FOB Tillman , Afghanistan ( CNN ) -- 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 did n'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 .
An Pakistani commander accuses Afghan 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 U.S. four times since June 2011 . A U.S. official says they try to liaise with Pakistani military forces first .