Updated at 11:37 a.m. Eastern.
A bomb killed three U.S. soldiers and partly destroyed a girls' school in northwest Pakistan on Wednesday in an attack that drew attention to a little-publicized American military training mission in the al Qaeda and Taliban heartland.
They were the first known U.S. military fatalities in Pakistan's lawless tribal regions near the Afghan border and a major victory for militants who have been hit hard by a surge of U.S. missile strikes and a major Pakistani army offensive.
The blast also killed three schoolgirls and a Pakistani soldier who was traveling with the Americans. Two more U.S. soldiers were wounded, along with more than 100 other people, mostly students at the school, officials said.
The attack took place in Lower Dir, which like much of the northwest is home to pockets of militants. The Pakistani army launched a major operation in Lower Dir and the nearby Swat Valley last year that succeeded in pushing the insurgents out, but isolated attacks have continued.
The Americans were traveling with Pakistani security officers in a five-car convoy that was hit by a bomb close to the Koto Girls High School.
There were conflicting reports about the source of the blast. Some officials said it was a roadside bomb detonated by remote control, while others claimed it was a suicide car bomb.
"It was a very huge explosion that shattered my windows, filled my house with smoke and dust and also some human flesh fell in my yard," said Akber Khan, who lives some 50 yards from the blast site.
The explosion flattened much of the school, leaving books, bags and pens strewn in the rubble.
"It was a horrible situation," said Mohammad Siddiq, a 40-year-old guard at the school. "Many girls were wounded, crying for help and were trapped in the debris."
Siddiq said the death toll would have been much worse if the blast had occurred only minutes later because most of the girls were still playing in the yard and had not yet returned to classrooms, some of which collapsed.
"What was the fault of these innocent students?" said Mohammed Dawood, a resident who helped police dig the injured from the debris.
The same school was damaged by a militant attack last year.
The soldiers were part of a small contingent of American soldiers training members of the paramilitary Frontier Corps, Pakistan's army and the U.S. Embassy said. The mission is trying to strengthen the ill-equipped and poorly trained outfit's ability to fight militants.
The soldiers were driving to attend the inauguration of a girl's school, which had been renovated with U.S. humanitarian assistance, the embassy said in a statement. The school that was ravaged by the blast was not the one where the convoy was heading, security officials said.
The explosion hit the vehicle in which the Americans were traveling along with members of the Frontier Corps, suggesting the attackers were targeting the Americans, according to Amjad Ali Shah, a local journalist traveling with the convoy to cover the school opening.
The attack will highlight the presence of U.S. troops in Pakistan at a time when anti-American sentiment is running high. U.S. and Pakistani authorities rarely talk about the American training program in the northwest out of fear it could generate a backlash.
Despite the presence of tens of thousands of U.S. forces in neighboring Afghanistan, Pakistan does not permit American troops to conduct military operations on its soil.
In a statement, the U.S. Embassy said three American military personnel were killed and two were wounded in the bombing. The Pakistani government condemned the attack in a statement that referred to the dead Americans only as U.S. nationals.
The last American killed in an attack in Pakistan was an American aid worker in the northwestern city of Peshawar in 2008.
Pakistani leaders are under intense pressure from many in their public to end all cooperation with the U.S. government, and senior politicians have frequently said no American soldiers would be welcome in combat roles inside the country.
Interior Minister Rehman Malik recently admitted to the media that U.S. military trainers were spread out across various locations throughout the country, but the country's leading Islamic political party was quick to seize on Wednesday's news, calling it evidence of the "ambiguity surrounding the presence of U.S. military and intelligence in Pakistan."
"I think it is about time that this air of deception comes to an end," Khurshid Ahmed, a senior leader of Jamaat-i-Islami, told CBS News' Farhan Bokhari in a telephone interview. It was a message clearly meant to apply more pressure from inside the country on Jamaat's political opponents, namely President Asif Ali Zardari, an ally to Washington.
The bodies of the slain American soldiers, along with the two injured personnel, were brought to the Shifa International hospital in Islamabad following the attack, reports CBS News' Maria Usman. The injured were being treated there, but due to a heavy U.S. security presence at the hospital the media was being kept away.
The blast hit a convoy close to the girls' school, celebrating its opening in the Shahi Koto area of Lower Dir district. CBS News correspondent Mark Phillips reports Lower Dir has been targeted in recent months in missile strikes by U.S. drone aircraft.
Two Pakistani reporters traveling in the same convoy as the Americans said that Pakistani military guides referred to the foreigners traveling with them as journalists. Initial reports of the attack, which proved incorrect, said four foreign journalists had been killed.
Mohammad Israr Khan, who works for Khyber TV, said two of the foreigners were wearing civilian clothes, not uniforms or traditional Pakistani dress.
"When our convoy reached near a school in Shahi Koto, I heard a blast," Shah, the journalist said. "Our driver lost control and something hit me and I fell unconscious."
Later, the bodies of three foreigners and two injured were flown by helicopter to Islamabad and then taken to the city's Al-Shifa hospital, said a doctor there who asked his name not be used citing the sensitivity of the case. One of the injured had minor head wounds and the other had multiple fractures.
He said Pakistani army and intelligence officers were present and not allowing visitors into the building.
Lower Dir shares a border with Afghanistan and with the Swat Valley, a region the army last year retook from militant control in an offensive. As part of its offensive against militants in Swat, the Pakistani army has carried out operations in Lower Dir.
U.S. troops have been training Pakistan's Frontier Corps since at least 2008. The corps is a major force in the northwest, but they have long been under-equipped and under-trained, making them a feeble front line against militants.
The training program was never officially announced, a sign of the sensitivity for the Pakistan's government in allowing U.S. troops on its territory. Frontier Corps officials have said the course included classroom and field sessions. U.S. officials have said the program is a "train-the-trainer" program, and that the Americans are not carrying out operations.