Islamabad, Pakistan (CNN) -- Tensions mounted Tuesday when NATO helicopters flying in eastern Afghanistan fired across the border into Pakistan after being fired on twice, according to a NATO official.

NATO's International Security Assistance Force later received reports that two Pakistani soldiers were wounded.

Two coalition helicopters supporting operations at U.S. Forward Operating Base Tillman in Afghanistan were fired upon from the Pakistani side of the border, said the NATO official, who did not want to be identified because the information has not yet been released publicly. After being fired upon a second time, the helicopters returned fire, the official said.

Tillman is a few kilometers from the Pakistan border.

The NATO-led force issued a brief official statement saying the alliance is "aware of the incident and is assessing it to determine what happened."

Pakistani officials confirmed that two soldiers were wounded. The Pakistani military said it has lodged a "strong protest" and called for a meeting of the two sides.

A NATO spokeswoman offered little detail about the incident.

"We're aware of a cross-border incident. We're still assessing the situation," said Lt. Commander Kaye Sweetser, a spokeswoman for the coalition troops.

Pakistani intelligence officials said the incident started when a NATO fighter jet entered Pakistani airspace near the border with Afghanistan. Pakistani troops began firing at the jet from the ground, said the officials who did not want to be named because they were not authorized to speak to the media.

The jet retreated and then returned, accompanied by helicopters, and fired on a military check post in a village in North Waziristan, the officials said.

The incident comes at a time of increased tension between Pakistan and the United States. Pakistani officials have bristled over an unauthorized U.S. raid into Pakistan to kill al Qaeda leader Osama bin Laden. U.S. drone strikes in Pakistan have also been a source of concern, as Islamabad says the strikes have killed civilians.

The Pakistani parliament recently condemned the raid and adopted a resolution calling for a review of its counterterrorism cooperation agreement with the United States. The resolution also ordered the immediate end of drone attacks that targets militants in a tribal region of Pakistan near the Afghan border.

U.S. Sen. John Kerry has been visiting Pakistan this week and said Pakistan on Tuesday will return the tail of a U.S. helicopter damaged during the bin Laden raid -- a move aimed at improving cooperation between the two nations.

The helicopter crashed during the May 2 raid of the al Qaeda leader's compound. Navy SEALs were able to destroy much of it, but the tail remained largely intact. In photos of the wreckage, aviation experts said they saw several telltale signs of stealth technology.

Kerry said Monday that the United States need not apologize to Pakistan for the raid but said it's important the countries find a way to heal their relationship.

CNN's Christine Theodorou and Nasir Habib contributed to this report.