Three US soldiers were among seven people killed today in a roadside bomb blast near a girls' school in north-west Pakistan, security officials said.
Another 70 people were hurt, many of them schoolgirls, according to a police spokesman.
One US soldier was also injured in the explosion, a Pakistan army spokesman later confirmed.
The troops were travelling with Pakistani security force members in the Lower Dir district as part of a US mission to train members of the paramilitary Frontier Corps to fight al-Qaida and Taliban militants.
If the deaths are confirmed by US authorities, they would represent a major victory for militants close to the Afghan border who have been hit hard in recent months by a surge in US missile strikes and a major Pakistani army offensive.
The blast hit a convoy close to a girls' school celebrating its opening in the Shahi Koto area.
At least seven people were killed in the attack, including a Pakistani soldier, officials said.
Around 70 people were wounded, among them many schoolgirls, according to police chief Mumtaz Zarin Khan.
Some officials said three schoolchildren also were among the dead.
Lower Dir shares a border with Afghanistan and with the Swat Valley, a region the army regained last year from militant control.
As part of its offensive against militants in the Valley, the Pakistani army has carried out operations in Lower Dir.
Local tribes have also set up militias to root out insurgents taking refuge in the area.