A US Army Apache helicopter fires a missile into a mountainside in the rugged Spira mountains in Khost province, Pakistan, Nov. 19, 2008.

Pakistani troops were engaging two American attack helicopters patrolling the volatile North Waziristan tribal region on Tuesday according state media reports.

In the incident two Pakistani soldiers were injured, in an exchange of fire with the two Apache helicopters from Afghanistan that intruded into Pakistan airspace further inflamed an already strained ties between Washington and Islamabad.

The helicopters entered into Pakistani air space at the Admi Kot border post in North Waziristan Agency in the early hours this morning, IndianExpress.com reports.

Pakistani military spokesman Maj Gen Athar Abbas, said the troops at the post fired upon the helicopters and, as a result of exchange of fire, two of our soldiers received injuries.

The Pakistan Army lodged a “strong protest” over the incident and demanded a flag meeting, he said. The incident occurred in Datta Khel region of North Waziristan, which has witnessed numerous missile strikes by US drones over the past few years.

The ISAF said it is investigating the incident further, but one senior U.S. official said it is “likely” the helicopter accidentally did pass into Pakistan. Campbell told ABC News that insurgents sometimes fire on NATO troops from near Pakistani outposts in hopes of drawing U.S. return fire.

Campbell said that communication between the American and Pakistani militaries had suffered in the backlash following the bin Laden operation, which has increased chances of such incidents on the border.

“You just have to be talking back and forth … so if something comes from Pakistan and somebody has fired, we can pull up our Pakistani counterparts and say, ‘Hey, we are getting fire from here.

We need you guys to go take care of that,'” he said. “So it’s very important to try to work that relationship.”