(Updates with U.S. denial)

ISLAMABAD, Sept 23 (Reuters) - A pilotless U.S aircraft crashed in northwest Pakistan's South Waziristan region on Tuesday, Pakistani news channels reported, but American officials denied the United States had lost any drones.

U.S. forces have launched a string of attacks with missiles fired by drones, and one ground assault, on militant targets in northwest Pakistan in recent weeks, infuriating Islamabad, which says the attacks violate Pakistani sovereignty.

Pakistani news reports said the drone came down near the border village of Angor Adda, where U.S. commandos launched a ground assault on Sept. 3 that Pakistani officials said killed 20 people, including women and children.

U.S. officials said no unmanned aircraft were lost or crashed.

"No such thing occurred," one senior U.S. official said.

A Pakistani intelligence officer in the region said he had no information but had heard claims by a Taliban faction that a drone had been shot down on the Afghan side of the border.

A Pakistani military spokesman declined to comment and reporters were unable to reach the remote area at night.

Dawn News, one of several channels reporting the incident, said security forces found wreckage of a drone near the village of Jalal Khel, 8 km (5 miles) from Angor Adda and 3 km (2 miles) from the border.

It said officials believed the drone had crashed, although Pashtun tribesmen claimed to have shot it down.

Earlier on Tuesday, U.S. President George W. Bush said in New York that visiting Pakistani President Asif Ali Zardari had spoken strongly about protecting Pakistani sovereignty.

Bush did not speak about the recent strikes.

"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," Bush said before meeting Zardari.

Pakistan's support is regarded as crucial both to the success of Western forces trying to stabilize Afghanistan and in the fight against al Qaeda.

But U.S. impatience has grown over what it sees as Pakistan's failure to eliminate the militant threat on its side of the Afghan border.

Pakistanis were outraged by the Sept 3 commando raid, the first known ground assault by U.S. troops into Pakistan, and the six-month-old civilian government issued a diplomatic protest.

Gen. Ashfaq Kayani 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 Pakistani security officials have reported troops firing on U.S. helicopters in recent days, forcing them to turn back to Afghanistan. U.S. officials have repeatedly dismissed the reports and said no helicopters had taken fire. (Reporting by Alamgir Bitani and Simon Cameron-Moore, Jeremy Pelofsky in New York and David Morgan in Washington; Writing by Robert Birsel; Editing by Tim Pearce and Peter Cooney)

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