WASHINGTON - The parading of U.S. air and naval power within view of the Korean Peninsula — first a few long-range bombers, then stealth fighters, then ships — is as much about psychological war as real war. The U.S. wants to discourage North Korea's young leader from starting a fight that could escalate to renewed war with South Korea.

Worries in Washington rose Tuesday with North Korea's vow to increase production of nuclear weapons material. Secretary of State John Kerry called the announced plan "unacceptable" and stressed that the U.S. is ready to defend itself and its allies. But he and other U.S. officials also sought to lower the rhetorical temperature by holding out the prospect of the North's reversing course and resuming nuclear negotiations.

At a joint news conference with visiting South Korean Foreign Minister Yun Byung-se, Kerry said the U.S. would proceed "thoughtfully and carefully" and in consultation with South Korea, Japan, China and others.

Defence Secretary Chuck Hagel, in a call late Tuesday to China's defence minister, called the North's development of nuclear weapons a "growing threat" to the U.S. and its allies.

Hagel, citing North Korea's pursuit of nuclear weapons and ballistic missiles in his phone conversation with Chang Wanquan, said Washington and Beijing should continue to co-operate on those problems, according to a Pentagon statement describing the call.

Michael Green, an Asia specialist at the Center for Strategic and International Studies, said it should be no surprise that North Korea is vowing to restart a long-dormant nuclear reactor and ramp up production of atomic weapons material.

"This is part of their protection racket," Green said in an interview. "I think the end state North Korea would like is that we, the U.S. in particular, but also China, Japan, South Korea, are so rattled by all this that we decide it's just better to cut a deal with them."

Tensions have flared many times in the six decades since a truce halted the 1950-53 Korean War, but the stakes are higher now that a defiant North Korea appears to have moved closer to building a nuclear bomb that could not only threaten the South and other U.S. allies in Asia but possibly, one day, even reach U.S. territory.

That explains, in part, why the U.S. is displaying military muscle to warn the North to hold its fire.

Washington also wants to leave no doubt that it has the South's back, and that Seoul should not act rashly. Nor does the U.S. want South Korea to feel compelled to answer the North's nuclear drive by building its own bomb.

"We are in the business of assuring our South Korean allies that we will help defend them in the face of threats," Pentagon press secretary George Little said, adding, "We are looking for the temperature to be taken down on the Korean peninsula."

Even without nuclear arms, the North poses enough artillery within range of Seoul to devastate large parts of the capital before U.S. and South Korea could fully respond. The U.S. has about 28,500 troops in the South, and it could call on an array of air, ground and naval forces to reinforce the peninsula from elsewhere in Asia and the Pacific.

In just the past few months, North Korea has taken a series of steps Washington deemed provocative, including an underground nuclear test in February. In December the North Koreans launched a rocket that put a satellite into space and demonstrated mastery of some of the technologies needed to produce a long-range nuclear missile. And several weeks ago, the North threatened to pre-emptively attack the U.S.

Bruce Bennett, a specialist in North Korean affairs for the RAND Corp., said he believes much of the recent taunting from North Korea reflects turmoil among the ruling elite in Pyongyang. He cited unusually high turnover among senior officials during the 15 months that Kim Jong Un — grandson of the nation's founder — has been the top leader.

"I think with the purges going on, he's got some instability that is generally not being recognized" outside of North Korea and that may be pushing Kim to take a more confrontational stance, Bennett said in an interview. "He's trying to be blustery to make it appear that he's really in control, he's really strong and he can defeat us."

In response, the Pentagon announced it would beef up missile defences based on the U.S. West Coast, and it highlighted over a period of days the deployment of B-52 and B-2 bombers, as well as two F-22 stealth fighters, to South Korea as part of an annual U.S.-South Korean exercise called Foal Eagle, which lasts through April.

On Tuesday, officials said the Navy was keeping the USS Decatur, a destroyer armed with missile defence systems, in the vicinity of the Korean Peninsula for an unspecified period instead of continuing its journey back to the U.S. after a Mideast deployment. And they said a similar ship, the USS McCain, had been shifted slightly to the waters off the southwest coast of the Korean Peninsula as a further precautionary move.

North Korea has been an enigma to most outsiders since it was founded by Kim Il Sung in 1948. The United States has often misjudged the North's political path. After the founding Kim died in 1994, for example, U.S. intelligence officials said they believed his successor son, Kim Jong Il, would be more accommodating to the West.

"Flaky as he may be, (Kim Jong Il) nevertheless ... realizes the only way they're going to extricate themselves from the shambles that their economy is in now is to get outside help," James R. Clapper Jr. told a congressional panel in January 1995. Clapper was director of the Defence Intelligence Agency at the time; today he is President Barack Obama's most senior intelligence adviser as director of national intelligence.

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AP broadcast correspondent Sagar Meghani contributed to this report.

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Follow Robert Burns on Twitter at http://www.twitter.com/robertburnsAP

Loading Slideshow...
  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un greets military personnel at a long-range artillery sub-unit of KPA Unit 641 during his visit to front-line military units near the western sea boarder in North Korea near the South's western border island of Baengnyeong. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un waves at military officers after inspecting the Wolnae Islet Defense Detachment, North Korea, near the western sea border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, confers with military officers at a long-range artillery sub-unit of KPA Unit 641 during his visit to front-line military units near the western sea border in North Korea near the South's western border island of Baengnyeong. (AP Photo/KCNA via KNS)

  • In this undated photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, second left, watches sturgeons in a pond at the Ryongjong Fish Farm in South Hwanghae, southwestern North Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un rides on a boat, heading for the Wolnae Islet Defense Detachment, North Korea, near the western sea border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, third left, looks at South's western border island of Baengnyeong during his visit to the Wolnae Islet Defense Detachment, North Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, is welcomed by military personnel at a military unit on Jangjae islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, walks with military personnel as he arrives for a military unit on Mu Islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, uses binoculars to look at the South's territory from an observation post at the military unit on Jangjae islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, with military officials, gets a ride on a boat on his way to a military unit on Jangjae Islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) meeting with family of a soldier as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) waving to soldiers from a wooden boat as he inspects the Mu Islet Hero Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) meeting with family of a soldier as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting Jangjae Islet near from South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting the Mu Islet Hero Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) posing with soldiers and their family members as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)