A prisoner killed two US Marines in southern Afghanistan after escaping a prayer room and grabbing a rifle, NATO said Monday. The alliance said the gunman was later shot dead and that the incident, on Saturday, was under investigation. 'The prisoner escaped a room where he was observing prayer time, acquired a rifle and subsequently engaged Afghan and coalition forces. The Marines were killed while trying to subdue the prisoner,' said NATO in a statement. Another NATO soldier was killed by a bomb in the south on Monday, adding to a toll of victims of explosives laid by insurgents which is rising almost daily. Eight foreign troops were killed over the weekend, two by the prisoner and six by bombs -- the Taliban's choice weapon in their southern heartlands of Helmand and Kandahar provinces, where 30,000 international troops are deployed. The latest death brings the overall number of foreign troops killed in Afghanistan this year to 426, compared to 520 for all of 2009, according to an selfnews tally based on that kept by the independent icasualties.org website. In the southeast, clashes broke out when insurgents attacked three NATO combat posts, the coalition force said. The troops came under attack in the rural province of Paktika, which shares a long, porous border with Pakistani areas troubled by Taliban militancy. NATO has bases in a few locations in the province close to the border. 'US and Afghan forces are thwarting insurgent attacks on three combat operation posts in... Paktika province today,' said NATO. The alliance said fighting was continuing and that it could not give further details on any casualties. NATO rarely releases statements about insurgent attacks on their bases unless the raids are significant or cause casualties. 