Pakistan's Taliban claimed responsibility for a bomb on Wednesday that killed three US soldiers outside a girls school in the northwest of the country and threatened more attacks on Americans.
Three children and a Pakistani soldier were also killed and 45 people, including 40 school girls, were wounded in the attack near Swat Valley, where the government mounted a crackdown nearly a year ago it said had cleared out Taliban militants.
The blast, triggered by a remote-controlled device, was a grim reminder of the resilience of Taliban militants determined to topple the government of President Asif Ali Zardari.
Security officials said earlier that the explosion killed four foreign aid workers.
The Taliban claimed responsibility for the bombing and threatened more attacks. "We will continue such attacks on Americans," Taliban spokesman Azam Tariq said.
Pakistan's Taliban have bombed markets, schools and military and police facilities despite major government security offensives that have destroyed some of their bases and US drone aircraft strikes that have killed their leaders.
The three US soldiers were part of a small unit that trains Pakistani Frontier Corps responsible for security in northwestern areas near the Afghan border seen as part of a global militant hub. They were on their way to attend the opening ceremony of a new school that had recently been renovated with US humanitarian assistance when the bomb exploded.
"The carnage at the school in Lower Dir clearly shows the terrorists' vision," said a US embassy statement. Militants have previously attacked US diplomats and facilities in Pakistan.
Pakistani foreign minister Shah Mehmood Qureshi also condemned today’s attack. He said "terrorism will never be allowed to succeed in its nefarious designs".
The appearance of Pakistani Taliban leader Hakimullah Mehsud, in a farewell video with the suicide bomber who killed seven CIA employees in Afghanistan in December, suggests the group poses an increasingly complex threat to Pakistan's security.
The United States is leaning heavily on long-time ally Pakistan to help it stabilise Afghanistan, a top foreign policy priority for President Barack Obama.
It wants Islamabad to eliminate al-Qaeda and Afghan Taliban militants who cross over the border to attack US- and Nato-led troops in Afghanistan.
But the nuclear-armed country is focused on fighting homegrown Taliban who have blown up dozens of girl’s schools and publicly flogged and executed those deemed immoral, an austere interpretation of Islamic rule they want to impose.
The possibility that some of his aides will be prosecuted under revived corruption charges and growing public frustrations with a sluggish economy and chronic power cuts have also piled pressure on Zardari.
US drone strikes in northwest Pakistan have intensified since the attack on the CIA in Afghanistan, but analysts say they are unlikely to pose a long-term danger to the Taliban, who seem to carry out suicide bombings at will.
The death toll from drone attacks last night has risen to 31, security officials said.