The incident could escalate tensions between the two allies as Islamabad has been criticised a series of recent US attacks targeting suspected al-Qaeda and pro-Taliban fighters inside Pakistan.

A number of sites have been hit by missiles from pilotless drones, while on September 3 a ground assault by US commandos killed 20 people, including women and children, according to Pakistani officials.

Lieutenant-Colonel Mark Wright, a US Pentagon spokesman, said it had "no reports of any loss of DoD [department of defence] drones".

The CIA declined to comment.

'No firing heard'

But senior Pakistani security official said that the drone, which did not disintegrate when it crashed, was believed to have been American.

"Tribesmen picked it up and then Pakistani security forces retrieved it. No firing was heard in the area so there is no question of it being shot down," he told the AFP news agency on condition of anonymity.

Dawn News, one of several Pakistani channels reporting the incident, said security forces had found the wreckage of the drone 8km from Angor Adda, near the village of Jalal Khel, and 3km from the border with Afghanistan.

Angor Adda was the site of the US commando raid in September.

'Sovereign right'

Before meeting Asif Ali Zardari, Pakistan's president, in New York on Tuesday, George Bush, the US president, said that Washington wanted to help in Pakistan.

"Your words have been very strong about Pakistan's sovereign right and sovereign duty to protect your country, and the United States wants to help," he said, referring to Zardari.

Robert Gates, the US defence secretary, said that Washington would continue to take military action in Pakistan and called for co-operation from the government in Islamabad.

"I think it is essential for Pakistan to be a willing partner in any strategy we have to deal with the threat coming out of the western part of Pakistan and the eastern part of Afghanistan," Gates said, expressing hope for "an even stronger partnership" with Zardari.

Pakistan's support is regarded as crucial to the success of US-led forces trying to stabilise Afghanistan and fight al-Qaeda in the region.

But Pakistanis have become increasing angered over the heightened use of drones and ground units in the area bordering Afghanistan.

Pakistanis were outraged by the US September 3 raid - the first known ground assault by US troops into Pakistan - and the six-month-old civilian government issued a diplomatic protest.

General Ashfaq Kayani, Pakistan's army chief, said foreign troops would not be allowed on Pakistani soil and Pakistan's sovereignty and territorial integrity would be defended at all costs.

Residents and some security officers said Pakistani troops fired on two US helicopters that crossed the border near Angor Adda a week ago, forcing them to turn back.

Pakistan and the United States denied the reports.