The overnight shooting by a U.S. army staff sergeant will undoubtedly reignite anti-American sentiment in the region, just beginning to dissipate following the burning of Qur'ans at a U.S. military base a month ago.
That incident set off widespread riots in Afghanistan, resulting in the deaths of six American servicemen and more than two dozen Afghan civilians, and disrupted the progress of the U.S. military withdrawal from the country.
It also eroded relations between the White House and Afghan President Hamid Karzai and threatened to complicate already contentious negotiations on a strategic partnership agreement to be implemented after U.S. combat operations end in 2014.
"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai, who has repeatedly demanded U.S. troops stop killing Afghan civilians, said in a statement Sunday on the latest incident.
Weeping villagers showed a photographer for The Associated Press 15 bodies, including women and children, and said they were all gunned down by the U.S. soldier.
As one woman spoke to reporters, she pulled back a blanket to reveal the body of a small child. The bodies of two other dead children were amid a pile of green blankets in the back of a truck.
Villagers drove the truck to the U.S. military base following the shooting to show Americans the victims.
Another villager, Gul Bashra, told reporters her two-year-old was among those gunned down.
"They killed a child who was two years old," she said. "Was this child a Taliban? Believe me, I have not seen a two-year-old Taliban yet. There is no Taliban here."
U.S. President Barack Obama expressed deep regret for the massacre that occurred in two separate villages — Balandi and Alkozai — located about a kilometre from an American military base in the Panjwai district of southern Kandahar province.
"This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan," he said, adding the soldier responsible will be held fully accountable.
Canadian troops spent a gruelling decade in Kandahar province, considered a Taliban stronghold, in a combat mission punctuated by violence and turmoil. More than 900 Canadian military advisers remain in Afghanistan since Canada's mission ended there last summer.
"The good work of our men and women in uniform, as well as the work of many fearless Canadian civilians, continues to bridge what Afghanistan is and what Afghanistan can be," Peter MacKay, Canada's defence minister, said in a statement on Sunday.
"That work will not be deterred by a random and cowardly act of violence."
MacKay called the massacre "deplorable," adding it "runs contrary to everything that the international mission aims to accomplish."
One of the five people wounded in the attack included a teenaged boy named Rafiullah, according to Karzai's statement. He was shot in the leg and reportedly spoke to the Afghan president over the telephone, describing how the American soldier entered his house in the middle of the night, woke up his family and began firing.
Asadullah Khalid, the government representative for southern Afghanistan and a member of the squad investigating the incident, told The Associated Press that the gunman went to three different houses and opened fire.
"When it was happening in the middle of the night, we were inside our houses," a resident of the village of Alkozai told the AP. "I heard gunshots and then silence and then gunshots again."
Twelve of the dead were from Balandi, said Samad Khan, a farmer who lost all 11 members of his family, including women and children. Khan was not in the village when the shooting occurred, only to return and find his entire family dead and burned. It was unclear how or why the bodies were burned.
"This is an anti-human and anti-Islamic act," Khan told reporters. "Nobody is allowed in any religion in the world to kill children and women."
Gen. John R. Allen, commander of the International Security Assistance Force (ISAF), offered his "profound regret and deepest condolences" to the victims and their families.
"I pledge to all the noble people of Afghanistan my commitment to a rapid and thorough investigation," he added in a statement.
In the aftermath of the shooting, the U.S. embassy in Kabul sent out a warning to American citizens in Afghanistan that "there is a risk of anti-American feelings and protests in coming days."
Simmering Afghan resentment of American forces has gone into a full boil in recent months.
A video posted online in January that showed U.S. soldiers urinating on the corpses of slain Afghan insurgents prompted outrage. The incident was condemned by the Pentagon.
In November, the ringleader of a rogue American military "kill team," charged with murder for shooting Afghan civilians for sport, was convicted and sentenced to life in prison by a military panel.
Joshua Foust, a fellow at the American Security Project, said the latest incident plays into fears in Afghanistan about the motives of American soldiers. Indeed, the Taliban released a statement accusing them of having murderous intentions.
"The so-called American peacekeepers have once again quenched their thirst with the blood of innocent Afghan civilians in Kandahar province," the statement read.
Foust pointed out there's already a belief in Afghanistan "that American soldiers are fighting a war in ways they really don't care for, to put it mildly," he said.
"This just adds to those concerns. And that will dramatically increase the amount of pressure that's present for Americans to withdraw as quickly as possible."
That pressure won't just come from the Afghans, Foust added, but stateside too.
"There's growing opposition to the war, in U.S. Congress and beyond, because it's bringing up all sorts of uncomfortable memories about Vietnam and questions, even though this is a somewhat freak event, about what kind of war are we waging if our soldiers are snapping this way."
American forces, Foust adds, have learned painful lessons in Afghanistan in the aftermath of the Canadian withdrawal.
"There was this tacit belief in the U.S. that Canada had somehow failed in Kandahar and now the adults were finally in charge. But they're finding out the problem wasn't Canada, the problem wasn't the U.K., the problem is Afghanistan."