A police officer looks at the remains of a vehicle destroyed by a bomb explosion in Lower Dir, Pakistan, on Wednesday. Three U.S. soldiers, three schoolchildren and a Pakistani soldier were killed. (Naveed Ali/Associated Press)Three American soldiers, a Pakistani soldier and three children from a girls' school were killed in a bomb blast in northwest Pakistan on Wednesday, officials said.

The bomb struck a security convoy as it travelled past the school celebrating its opening in the Lower Dir region, not far from the Afghan border.

At least 100 people were wounded, including two U.S. soldiers and dozens of schoolchildren, according to army officials and police chief Mumtaz Zarin Khan. The U.S. Embassy confirmed the American casualties.

The Pakistan Taliban claimed responsibility for the attack, according to a Reuters report.

The bomb destroyed much of the Koto Girls High 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."

The same school was damaged by a militant attack last year.

Girls' schools are a frequent target of the Taliban, who consider female education a violation of Islam. The Taliban banned education for girls and forced working women to return to their homes when the militant Sunni group controlled neighbouring Afghanistan from 1996 to 2001.

There were conflicting reports about the source of the blast. Some officials said it was a roadside bomb detonated by remote control, while others said it was a suicide car bomb.

An injured student is admitted to hospital Wednesday after a roadside bombing near a girls' school in Lower Dir in northwest Pakistan. (Rohullah Shakir/Associated Press)

Security forces targeted

The region where the blast occurred, near the Swat Valley, was thought to have been largely cleared of militants during last year's military offensive. The Pakistani government has been waging a war against militants believed to be primarily based in the country's tribal regions near the Afghanistan border, though the militants have launched attacks from many regions of the country, often against security and police forces.

The Pakistani army said Sunday it was investigating reports that Pakistani Taliban leader Hakimullah Mehsud had died of injuries suffered in a U.S. drone missile strike in mid-January. His death would be an important success for Pakistan, which has been battling the Pakistani Taliban, and the U.S., which blames Mehsud for a recent deadly bombing against the CIA in Afghanistan.

The U.S. soldiers killed and injured in the blast were part of a small group of American service members training the paramilitary Frontier Corps, the local security force in the region, officials said.

American training of Pakistani forces in the region has been ongoing since 2008 but has been little publicized, as Pakistan's government is sensitive to domestic criticism that it is too closely aligned with the United States in its effort to combat militant groups.

With files from The Associated Press