NATO helicopters from Afghanistan wounded two Pakistani soldiers in a cross-border attack Tuesday, officials said, triggering a "strong protest" from Islamabad as tensions with the US simmered after Osama bin Laden's death.

The two choppers opened fire in the direction of a restive tribal region in Pakistan's northwest after they were shot at, a Western military official in Kabul said, amid conflicting reports of the incident.

Pakistan's military said its troops fired on the helicopters after they violated Pakistan airspace.

The incident occurred just one day after US Senator John Kerry attempted to soothe a row with Pakistan's military and civilian leadership about the May 2 raid that killed bin Laden north of Islamabad.

The helicopter attack took place in Wacha Bibi, 50 kilometres (30 miles) west of Miranshah, the main town in North Waziristan tribal district, officials said.

Washington considers the tribal belt a hotbed of Al-Qaeda, where Taliban and other militants plot attacks on American troops, including those in the US-led international force based in Afghanistan, and on Western targets.

"Pakistan's army has lodged a strong protest and demanded a flag meeting," with NATO officials in Afghanistan, a statement issued in Islamabad said.

"Two NATO helicopters violated Pakistan airspace today at Admi Kot Post, North Waziristan, in the early hours of the morning.

"The troops at the post fired upon the helicopters and, as a result of exchange of fire, two of our soldiers received injuries."

The incident comes as relations between Islamabad and Washington remain tense following the commando raid that killed bin Laden, an attack that embarrassed and angered the Pakistani military and leadership.

The western military official in Kabul, who requested anonymity, told AFP that the two helicopters were in Afghanistan "in support of a forward operating base which was receiving fire from across the border of Pakistan".

"Upon arrival at the scene, one of the helicopters received fire from across the border but didn't immediately return fire. Upon receiving fire a second time, the helicopter returned fire," he added.

Pakistani authorities later informed ISAF (the US-led International Security Assistance Force in Afghanistan) that two soldiers had been wounded, he said.

A spokesman for the international military alliance in Afghanistan said ISAF "had reports of a possible incident".

"We are looking into it," Lieutenant-Colonel John L. Dorrian said.

The Pakistani military often accuses the NATO force in Afghanistan of violating Pakistan's airspace in the hunt for Taliban militants who launch attacks before fleeing back across the border into Pakistan.

The disputed, unmarked border area between Afghanistan and Pakistan has been described by United States President Barack Obama as the most dangerous place in the world — it is seen as the global headquarters of Al-Qaeda and the Taliban.

US officials have long pressed Islamabad to crack down on the Haqqani network and other militants based in North Waziristan, saying the insurgents exploit the area as a sanctuary to stage attacks on coalition forces in neighbouring Afghanistan.

Pakistan temporarily shut the main land route for NATO supplies into Afghanistan last September after officials accused NATO of killing Pakistani troops in another cross-border attack.

The northwest region is being targeted by a record number of US drone strikes, the number of which has doubled in the last year, with more than 100 strikes killing over 670 people, according to an AFP tally.

The CIA says the covert programme has severely disrupted Al-Qaeda's leadership.

US drone strikes inflame anti-American feeling in Pakistan, which has worsened since a CIA contractor shot dead two Pakistani men in a busy Lahore street in January.

Two US drone strikes targeting a militant compound and a vehicle in North Waziristan on Monday killed at least nine people.

Separately Tuesday, in Baluchistan province in Pakistan's southwest, security forces foiled a major suicide attack when they killed five militants — three of them women — equipped with suicide vests.

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