PESHAWAR - About 300 to 400 heavily armed militants crossed into Pakistans rugged northwest from Afghanistan on Wednesday and attacked a security checkpost, killing one policeman, police said, and a television station put the death toll at seven. Pakistani officials said the militants attacked from Afghanistan, targeting the Shaltalu checkpoint in a remote part of Upper Dir, near the border with Kunar province of Afghanistan. Securing the long, porous border that divides Pakistan and Afghanistan has been a major challenge for the two countries as they try to stop insurgents. Militants came from Afghanistan and attacked our forces. The operation is continuing, said Lieutenant General Asif Yasin Malik, the top military commander in Khyber Pakhtunkhwa. Qazi Jamil ur-Rehman, a senior police officer in the area, said the army was helping police after militants attacked the checkpost on three sides. We have sent reinforcements. Army helicopters are also shelling, he said. They are in the hundreds and very well armed with light and heavy weapons. The area is remote and surrounded by mountains, Rahim Gul, a police official told AFP by telephone from the nearby Barawal police station. Initial contact by radio had been lost, he said, but helicopter gunships had been mobilised to try to put down the attack, with around 40 local and tribal police resisting on the ground. A senior intelligence official in Peshawar said 300 to 400 militants had attacked from across the border. There was a firefight at the checkpoint and after the fighting we have had no contact with policemen there. We have received just one dead body of a policeman but we fear they have killed several policemen, he told AFP. They are resisting and are firing on police. We are facing problems because they are in the mountains. We have sent reinforcements. They are using both light and heavy weapons. We are engaging them from the air. Helicopters are shelling them, he added. They (militants) were in military uniform. They attacked the outpost and then went into forest there, Mahmood Ahmed, a police officer in the region, told Reuters by telephone. Police confirmed the death of one policeman but said seven coffins had been sent to the area, fearing that the death toll could rise. A private TV channel said seven police were killed in Wednesdays assault. Ahmed said all communication with the village had been cut off. Witnesses said heavy contingents of army and paramilitary troops have entered the area and helicopter gunships have been seen pounding suspected militant positions. At least 13 Pakistani security troops were killed in a similar cross-border raid by Taliban in the neighbouring Lower Dir district two months ago. The attack is the latest bloodshed following the killing of al Qaeda leader Osama bin Laden by US special forces in Pakistan on May 2. The Pakistani Taliban and affiliated groups have vowed to carry out attacks to avenge his death. However the latest violence seems to be a reaction to ongoing military offensives against insurgents along the border.