Islamabad - At least three people were killed and half a dozen injured on Friday as a US drone attacked a house in Pakistan's tribal area along the Afghan border.

A security official said a pilotless US drone fired missiles at a house in Mohmmad Khel village in North Waziristan.

More casualties were feared as some people were believed to be buried under the rubble, said the official, who spoke on condition of anonymity.

However, a spokesman from the Pakistan Army denied the report saying the US planes only hit targets on Afghan side of the border.

"The International Security Assistance Force had informed us beforehand about their operation along the border in Afghanistan," Major General Athar Abbas said.

The US attack came three days after two missiles fired from a US drone killed six people in North Waziristan, which is regarded as a haven for Taliban militants and members of the al-Qaeda terrorism network.

Having complained about Islamabad's inability to eliminate the hideouts of the Taliban, who launch cross-border attacks on international forces in Afghanistan, US forces have stepped up strikes on militant locations inside Pakistan, causing tension between the two countries.

Pakistani and US forces have officially admitted an exchange of fire at least once in recent weeks.

Prime Minister Yousaf Raza Gilani said on Thursday that the US "has to respect the sovereignty and integrity of Pakistan as an independent country. Nobody is allowed to violate the sovereignty of Pakistan." (dpa)