MIRANSHAH, Pakistan — 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 the country's airspace.

The incident took place just a day after US Senator John Kerry attempted to soothe a row with Pakistan's military and civilian leadership over the May 2 raid that killed bin Laden in a garrison city north of Islamabad.

The raid heaped pressure on Pakistan's government and security forces to explain how the Al-Qaeda chief lived undetected on its soil for what may have been years, and heightened US demands that they do more to fight militancy.

Late Tuesday Pakistani security forces announced the arrest of an Al-Qaeda operative called Muhammad Ali Qasim Yaqub -- alias Abu Sohaib Al Makki, believed to be a Yemeni national -- in the southern city of Karachi.

"According to preliminary investigations... (Al Makki) has been working directly under Al-Qaeda leaders along Pak-Afghan borders," the military's media wing said, calling the arrest a "major development in unravelling the Al-Qaeda network operating in the region".

Earlier, Islamabad issued a strongly worded statement following the helicopter attack.

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

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

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 targets in the West.

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 said.

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".

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

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 disputed, unmarked border area between Afghanistan and Pakistan has been described by US President Barack Obama as the most dangerous place in the world.

US officials have long pressed Islamabad to crack down on the Haqqani network -- blamed for some of the deadliest anti-US attacks in Afghanistan -- and other militants based in North Waziristan.

The northwestern region is the target of a record number of US drone strikes, which doubled last year from the year before, 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, but the strikes inflame anti-American feeling in Pakistan, which has worsened since a CIA contractor shot dead two Pakistanis 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.