The participation of the U.S. and five Arab nations marked an unexpected foreign policy victory as Obama plunges the U.S. deeper into a military conflict in the Middle East that he has reluctantly embraced.
"We are joined by our partners. America is proud to stand shoulder to shoulder with these nations," Obama said in an address from Washington. "This coalition makes it clear this is not just America's fight alone. Meanwhile, we will move forward with our plan supported by partisan majorities in Congress to train Syrian and Iraqi opposition fighters. Over 40 nations have offered to help to confront this terrorist threat."
The U.S. announced the strikes hours before Obama was due to arrive in New York for three days of talks with foreign leaders at the annual United Nations General Assembly. The co-operation by Arab partners provided a significant boost to Obama's efforts to build an international coalition to take on the Islamic State militants who have moved freely across the border between Iraq and Syria.
U.S. officials said Bahrain, Qatar, Saudi Arabia, Jordan and the United Arab Emirates participated in the strikes against Islamic State targets, as Obama significantly ramped up U.S. military involvement in Syria, a country that has been mired in a brutal three-year civil war.
"We will not tolerate safe havens for terrorists who threaten our people. We will do what is necessary to defend our country," Obama said.
U.S. military spokesman Lt.-Gen. William C. Mayville Jr. said during a briefing on Tuesday that the airstrikes in Syria were successful and "only the beginning."
The U.S. joint staff director of operations showed reporters slides "that highlight the precision of these missiles."
'Trying to disrupt their support bases'
"The intended target was the communications array on the roof of the building," Mayville said of one image. "The communications array was destroyed, while the building remained largely intact."
Mayville said Arab nations' role in the military action in Syria ranged from combat air patrols to strikes on specific targets.
"We are trying to disrupt their support bases," he said.
Pentagon officials said they did not coordinate the airstrikes in Syria with the regime of President Bashar al-Assad.
As Obama arrives in New York for the UN meetings, he'll face other crises that highlight the extraordinary range of challenges demanding U.S. attention across multiple continents. He'll speak at a high-level UN meeting on the Ebola outbreak in West Africa and meet with other leaders to discuss Russia's provocations in Ukraine.
The U.S. and partner countries launched airstrikes Monday night on targets in eastern Syria, and the U.S. undertook a separate, unilateral air attack on what it called an al-Qaeda affiliate elsewhere in Syria.
Several hours after the Pentagon announced the airstrikes against Islamic State targets, U.S. Central Command said American warplanes launched eight airstrikes "to disrupt the imminent attack plotting against the United States and Western interests" by a network of "seasoned al-Qaeda veterans" — sometimes known as the Khorasan Group — who have established a haven in Syria. It provided no details on the plotting.
Central Command said that separate bombing mission was undertaken solely by U.S. aircraft and took place west of the Syrian city of Aleppo. It said targets included training camps, an explosives and munitions production facility, a communication building and command and control facilities.
Syrian refugees pour into Turkey
More refugees were pouring into Turkey from Syria on Tuesday as the airstrikes were being carried out.
An official from Turkey's crisis management agency told the AP Tuesday that the number of refugees who reached Turkey escaping an Islamic State group's advance in the past few days now stands at around 150,000.
The UN refugee agency says that along with the Turkish government, it had made contingency plans for hundreds of thousands who might cross the border, but could not predict a number.
The bulk of the president's agenda in New York will focus on bolstering the coalition that will take on the Islamic State militants. While the U.S. has been carrying out strikes against militant targets in Iraq, Monday's strikes were the first in Syria.
U.S. seeks to rally more partners
Obama has insisted the U.S. would not be alone in trying to root out the Islamic State group, but the public commitments from allies had been few and far between. Before Monday night's strikes, only France had committed to airstrikes in Iraq, and Saudi Arabia had volunteered to host U.S.-led training missions for Syrian rebels.
Even with the actions from Arab nations, the U.S. is seeking to rally other partners for future cooperation. High on the list is Turkey, a U.S. ally and NATO member.
"Nations like Turkey have their own clear, vested personal interest in confronting the threat that's posed by ISIL," White House spokesman Josh Earnest said, referring to the Islamic State by one of its many acronyms. "All of the mayhem and havoc that ISIL is wreaking in Iraq and in Syria is right on Turkey's doorstep. And it's certainly not in their interest for all that instability and violence to be occurring so close to their border."
Obama is not scheduled to have a formal bilateral meeting with Turkish President Recep Tayyip Erdogan in New York, though the leaders are likely to have some interaction on the sidelines of the discussions.
Obama will hold his first meeting with new Iraqi Prime Minister Haider al-Abadi, who took office in August amid a U.S.-led push to make his country's government more inclusive to Sunnis who had turned against Baghdad and toward the Islamic State group.
The president will also chair an unusual UN Security Council meeting Wednesday at which members are expected to adopt a resolution that would require all countries to prevent the recruitment and transport of would-be foreign fighters preparing to join terrorist groups such as the Islamic State. However, Obama administration officials have acknowledged that UN resolutions can be notoriously difficult to enforce.