Three Western aid workers are among at least 10 people killed in an attack on a convoy heading to a girls' school in north-west Pakistan, officials said.

At least 70 people, including 63 school girls, were injured as the convoy was hit as it passed another girls' school in Lower Dir, near the Afghan border.

The attack comes amid a major government offensive against Taliban militants in the area.

The Taliban has frequently targeted markets, schools and security agencies. The BBC's Mark Dummett in Islamabad says the convoy was heading to the inauguration ceremony of a newly-built girls' school, but that the blast occurred near a different school in Koto, a heavily populated village along the route.

At least three of the dead were school girls, police said, adding that security guards and three local journalists were also among the wounded.

The convoy was on its way to Maidan, an area of Lower Dir district in North West Frontier Province, which is the base of a pro-Taliban cleric Maulana Sufi Mohammad, and a stronghold of Taliban militants.

Prime Minister Yousuf Raza Gilani has condemned the attack and ordered an investigation, the AFP news agency reports.

Citing a local police official, AFP said the school had been blown up by militants in January 2009. It was due to be formally re-opened after being rebuilt with the help of foreign aid.

Last year, Pakistani troops carried out a major offensive to crush the Taliban in Lower Dir and the neighbouring districts of Swat and Buner.

Hundreds of people have died and several thousand displaced by the fighting. Thanks to BBC