NATO helicopters from Afghanistan intruded into northwest Pakistan and attacked a military check post near the border Saturday, killing up to 14 troops and wounding seven, Pakistani intelligence officials said.

The attack comes as relations between the United States and Pakistan, its ally in the war on terror, are already strained following the killing of al Qaeda leader Osama bin Laden by U.S. special forces in a secret raid on the Pakistani garrison town of Abbottabad in May. Pakistan called that raid a flagrant violation of its sovereignty.

A Pakistani military spokesman confirmed Saturday's pre-dawn attack in the tribal region of Mohmand and said casualties had been reported, but gave no details.

NATO helicopters carried out an unprovoked and indiscriminate firing on a Pakistani check post in Mohmand agency, casualties have been reported and details are awaited, the spokesman told Reuters.

Two intelligence officials in the region said that up to 14 Pakistani troops had been killed and seven wounded in the attack on the Salala check post, about 2.5 km (1.5 miles) from the Afghan border.

The attack took place around 2 a.m. (2100 GMT) in the Baizai area of Mohmand, where Pakistani troops are fighting Taliban militants.

A spokesman for the NATO-led International Security Assistance Force in Kabul said the coalition there was aware of an incident and was gathering more information.

The incident occurred a day after U.S. General John Allen met Pakistani Army Chief of Army Staff General Ashfaq Pervez Kayani to discuss border control and enhanced cooperation.

The Afghanistan-Pakistan border is often poorly marked, and differs from maps by up to 5 miles in some places.

A similar incident on Sept 30, 2009, which killed two Pakistani troops, led to the closure of one of NATO's supply routes through Pakistan for 10 days.

NATO apologised for that incident, which it said happened when NATO gunships mistook warning shots by the Pakistani forces for a militant attack.

Pakistan is a vital land route for 49 percent of NATO's supplies to its troops in Afghanistan, a NATO spokesman said.

The attack will likely worsen U.S.-Pakistan relations, already at one of their lowest points in history, following a tumultuous year that saw the bin Laden raid, the jailing of a CIA contractor, and U.S. accusations that Pakistan backed a militant attack on the U.S. Embassy in Kabul.

An increase in U.S. drone strikes on militants in the last few years has also irritated Islamabad, which says the campaign kills more Pakistani civilians in the border area than activists. Washington disputes that, but declines to discuss the drone campaign in detail.

This is an attack on Pakistan's territorial sovereignty, said Masood Kasur, the governor of Pakistan's northwestern Khyber-Pakhtunkhwa province. Such cross-border attacks cannot be tolerated any more. The government will take up this matter at the highest level and it will be investigated.

(Additional reporting by Saud Mehsud; Writing by writing by Augustine Anthony and Chris Allbritton; Editing by Ron Popeski)