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ISLAMABAD, Sept 22 (Reuters) - Pakistani troops fired on two U.S. helicopters that intruded into Pakistani airspace on Sunday night, forcing them to turn back to Afghanistan, a senior Pakistani security official said on Monday.

It was the second such incident in a week, and reflects frayed relations with the United States over Pakistan’s failure to act more forcibly against Islamist fighters in the tribal lands bordering Afghanistan.

The number of missile attacks by U.S. drone aircraft in the remote tribal areas has multiplied in recent weeks.

The helicopters violated the border in the area of Lowara Mandi, 40 km (25 miles) west of Miranshah, the main town in the North Waziristan tribal region, at around 9 p.m. on Sunday, the official said, speaking on condition of anonymity because of the sensitivity of the subject.

There was no official confirmation.

“We don’t have any information on border violation by the American helicopters,” Major Murad Khan, a military spokesman, said.

Residents said drones had been flying overnight and early on Monday over Miranshah but did not attack.

“It’s really so scary, we just can’t sleep when you hear them flying,” said Zia-ur-Rehman, a resident of Hamzoni village near Miranshah.

Relations became strained between the allies in the war on terrorism after U.S. commandos raided a border village in South Waziristan and killed 20 people, including women and children, on Sept. 3.

Pakistanis were outraged by the raid and the six-month-old civilian government issued a diplomatic protest.

It was the first overt incursion by U.S. ground troops into Pakistani territory, though the U.S. military has conducted numerous missile strikes against al Qaeda and Taliban targets in Pakistan’s tribal lands.

ZARDARI TO MEET BUSH

The U.S. action prompted army chief General Ashfaq Kayani to issue a statement saying that foreign troops would not be allowed on Pakistani soil and Pakistan’s sovereignty and territorial integrity would be defended at all costs.

Pakistani troops fired on two U.S. helicopters that crossed the border at the same village, Angor Adda, a week ago, again forcing them to turn back, according to residents and security officers.

Pakistan and the United States issued denials.

Pakistani President Asif Ali Zardari is scheduled to meet President George W. Bush on Tuesday in the United States, and is also due to attend the U.N. General Assembly.

Pakistan’s support is regarded as crucial to the success of Western forces trying to stabilise Afghanistan, and in the fight against al Qaeda.

Admiral Mike Mullen, the chairman of the joint chiefs of staff, held talks in Islamabad with top Pakistani officials last week. Mullen “reiterated the U.S. commitment to respect Pakistan’s sovereignty”, the U.S. embassy said in a statement.

The United States is Pakistan’s largest donor, and the dissonance between them has been a factor pushing down Pakistani shares and the rupee because Pakistan needs an inflow of dollars to rebuild rapidly depleting foreign currency reserves to prop up its ailing economy. (Writing by Simon Cameron-Moore; Editing by David Fogarty)