Despite severe protest and warnings by Pakistan, the Nato helicopters Thursday shelled a paramilitary checkpost and killed three soldiers and injured three in a cross border attack in Kurram Agency, security officials said. Meanwhile Pakistan has blocked supplies for NATO troops fighting in Afghanistan after an airstrike. The Nato choppers violated Pakistan's airspace at 5:00am and pounded an FC checkpost at , killing three FC men and injuring as many at Mand-To-Kandao, Kurram Agency. "It was an unprovoked attack that took place early Thursday morning. Nato helicopters entered our airspace and targeted a paramilitary checkpost killing three soldiers and wounding three others," a senior security official told mediamen. Meanwhile the Nato Spokesman in Afghanistan has said we are investigating the attack on Pakistani check post. ISAF in a statement said that coalition forces did not enter the Pakistani side and they only retaliated to firing of militants in Afghan province of Paktia District Dandpatan. The ISAF spokesman said however on complaint of Pakistan we are investigating the incident. This has been Nato 's third attack in Pakistani territory within a week. Meanwhile Pakistan has blocked supplies for NATO troops fighting in Afghanistan after an airstrike killed three Pakistani security personnel, Umair Khan, a local government official in Khyber Agency told media by telephone. Two soldiers were also injured in the strike, Kamal Khan, deputy district officer for the Kurrum Agency said by telephone.