NATO helicopters from Afghanistan intruded into northwest Pakistan and attacked a military check post near the border on Saturday, killing eight troops and wounding four, 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.

A Pakistani military spokesman confirmed the 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 eight Pakistani troops had been killed and four 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 02:00 a.m. (2100 GMT) in the Baizai area of Mohmand, where Pakistani troops are engaged in 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.

(Reporting by Saud Mehsud and Shams Momand; Additional reporting and writing by Augustine Anthony; Editing by Chris Allbritton and Nick Macfie)