Story highlights An Afghan commander accuses Pakistani soldiers of firing across the border

Pakistani military spokesman confirms U.S. troops have fired across border

NATO says a base near the border has fired into Pakistan four times since June

A U.S. official says they try to liaise with Pakistani military forces first

U.S. troops have fired into Pakistani territory at least four times in the last 10 months in cross-border skirmishes that they say are in response to shelling from inside Pakistan, CNN has learned.

The revelation is likely to stoke already tense relations between Pakistan and the United States, which hit a new low after a NATO airstrike last year killed 24 Pakistani soldiers along the volatile border.

While the Taliban and Haqqani network, an Afghan militant group operating from Pakistan's Waziristan province, are generally believed responsible for cross-border attacks against troops, an Afghan army commander says Pakistani soldiers opened fire on him and his men as recently as April 14.

"When we went near the border, we were attacked with an anti-aircraft gun and mortars by the Pakistani army from their checkpoints. We are not only attacked from the Pakistani soil, but we are also attacked by the Pakistani army," Masoud Karimi, an Afghan army commander, told CNN.

"We just saw them with our own eyes that the Pakistani soldiers were firing at us and on the same day we reported to the Americans. But the Americans told us that it was from the other side of the border and they didn't have the permission to conduct operations ... there," he said.

Army Lt. Col. Jimmie Cummings, a spokesman for the NATO-led International Security Assistance Force, confirmed Afghan soldiers were involved in a cross-border clash that day. He did not specify who opened fire upon the Afghans.

U.S. troops do, however, sometimes fire back into Pakistan, as confirmed during a recent visit by CNN to Forward Operating Base Tillman in Afghanistan's rugged Paktika province.

The commander at FOB Tillman, located just a few kilometers from the Pakistan border, said he has fired across the border in response to being shelled from inside Pakistan.

Army Capt. Charles Seitz said he did not know the exact number of times he returned fire.

But when pressed by CNN, and asked whether it was correct to say that he had fired into Pakistan more than five times, he responded: "That sounds accurate."

The outpost is one of a handful along the Afghan-Pakistan border, and the captain's admission may indicate more cross-border violence than previously publicized.

Cummings, the ISAF spokesman, said troops at Tillman have fired into Pakistan four times since June 2011.

Pakistan is aware of the cross-border skirmishes.

Gen. Athar Abbas, a spokesman for the Pakistani military, said he did not know the exact number of times FOB Tillman fired into Pakistan, but that four sounded accurate.

A U.S. official, speaking on condition of anonymity because of the sensitivity of the issue, said they try to liaise with Pakistani military forces first.

But that communication does not always happen, Abbas said.

He said that most of the time U.S. officials or NATO inform Pakistan before firing across the border, but sometimes they do not and Pakistani military checkpoints are hit.

Abbas said he was unaware of the April 14 cross-border clash.

FOB Tillman is named for fallen Army Spec. Pat Tillman -- the Arizona Cardinals linebacker turned soldier who was killed in a friendly fire incident in 2004 in Afghanistan, near the border.

Relations between the United States and Pakistan -- complicated at the best of times -- have become especially tense in the past year.

The deaths of 24 Pakistani soldiers in November added to anger already felt by Pakistanis over the U.S. raid that killed Osama bin Laden at a compound in Pakistan in May, and continued American drone strikes on targets in the nation.

Former chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, accused Pakistan's intelligence service of protecting the Haqqani network, calling it a "veritable arm" of the Pakistan spy agency. As a result, he said Pakistan bore some responsibility for the attacks.

The combined effect was a chilling of Pakistani cooperation with the United States.