A suspected US unmanned spy plane crashed Tuesday in a troubled Pakistani tribal area bordering Afghanistan where Al-Qaeda and Taliban extremists are active, officials said.

The crash in the South Waziristan region came amid growing tensions between Washington and Islamabad over US military incursions into Pakistani territory and missile strikes on Islamist militant targets.

The Pentagon said it had no report of any crash, while the CIA declined to comment.

"We have no reports of any loss of DoD (Department of Defense) drones," said Lieutenant Colonel Mark Wright.

The cause of the crash was not immediately clear, although some residents said that tribesmen shot at the aircraft before it came down near the frontier village of Angoor Adda.

"A pilotless spy plane, we believe it is US, crashed in Pakistani territory but it did not disintegrate. Tribesmen picked it up and then Pakistani security forces retrieved it," a senior Pakistani security official told AFP.

"No firing was heard in the area so there is no question of it being shot down," he said.

Residents however said that the plane was shot down.

"The tribesmen fired at the drone and it fell out of the sky," one resident told AFP by telephone, asking not to be named for fear of reprisals.

There was no immediate comment from the US-led coalition in Afghanistan. The US Central Intelligence Agency is also known to operate pilotless aircraft in the restive border region.

The incident comes just two days after Pakistani officials said that their troops had opened fire to repel two US helicopter gunships that crossed into Pakistani airspace in neighbouring North Waziristan.

South Waziristan was the scene on September 3 of the first officially acknowledged operation by US forces on Pakistani soil, in which 15 people were killed. Pakistan lodged a strong protest over the incident.

There have also been several recent missile strikes by American drones in the region.

Earlier US President George W. Bush held his first ever talks with new Pakistani counterpart Asif Ali Zardari over the tensions between the two countries.

The US president expressed his "deepest condolences" over Saturday's suicide attack on the Marriott Hotel in Islamabad, which left at least 60 people dead, and declared: "We stand with you."

The chairman of the US joint chiefs of staff, Admiral Michael Mullen, said during a visit to Islamabad on September 17 that Washington respects Pakistan's sovereignty.

Hours after his visit, at least five people were killed when four missiles fired by suspected US drones struck a compound in South Waziristan.