The US department of defence has denied that its helicopters flew into Pakistani airspace above its troubled border region with Afghanistan.

Pakistani intelligence officials say US helicopters flew into North Waziristan, an area considered to be home to tribal fighters and groups who back the Taliban, but returned to Afghanistan after troops and tribesmen opened fire.

"There was no such incursion, there was no such event, Colonel Gary Keck, a Pentagon spokesman, said on Monday.

Two helicopters flew over North Waziristan on the border with Afghanistan near Lwara Mundi village late on Sunday, two local intelligence officials said on condition of anonymity.

Reports of US forces carrying out operations inside Pakistan have drawn strong protests from Pakistani leaders.

Asif Ali Zardari, the Pakistani president, told NBC news that the US was forbidden from allowing any operations without permission.

"If the American troops are coming in without letting us know, without the Pakistani permission, they are violating the United Nations charter."

Series of raids

US commandos crossed the border into Pakistan on September 3 to attack a suspected al-Qaeda target that officials said was contributing to violence in Afghanistan.

Five people were killed when unmanned drones fired missiles into a village in South Waziristan last Wednesday evening, according to Pakistani officials.

And also last week, Pakistani troops were reported to have fired in the air to repel a US ground incursion into South Waziristan.

But there has been frustration in Washington over Pakistan's apparent slowness to act against fighters on its territory.

US officials say that armed fighters are using the tribal areas in northwestern Pakistan as a base to launch attacks on Afghan and US forces operating across the border.