Fighters have killed a police captain at a checkpoint and gunned down an Iraqi guardsman in a billiard hall in separate attacks against US-backed forces in northern Iraq. Police Captain Basam Ali Ahmad died of wounds a day after being shot on Thursday as he manned a checkpoint in Samarra, police said. Despite US-led forces mounting a major offensive against fighters in the city two months ago, frequent attacks continue. Further to the north, in the oil-refining town of Baiji, Ziyad Tariq, an off-duty member of the Iraqi National Guard, was shot dead on Friday by men who burst into a billiard hall armed with automatic rifles, police and hospital staff said. Iraq's new US-trained security forces are prime targets for fighters opposed to the interim government. **Two US soldiers killed as helicopters collide*** Two US soldiers were killed and four wounded when two helicopters collided at a military airfield in northern Iraq, the US army said. A Black Hawk helicopter was on the ground at Mosul airfield when an Apache crashed into it, an army spokesman said. The cause of the accident was under investigation, he said, adding that the wounded soldiers had all been returned to duty. He gave no further details. **Convoys targeted*** In other incidents on Friday a man and a woman, both civilians, were injured near Baquba when a roadside bomb detonated as a convoy of Iraqi National Guard vehicles passed by. No guardsmen were injured, eyewitnesses said. A similar bomb also struck a US military convoy north of the city but caused little damage. An explosion also caused a large fire near Baquba on an oil pipeline that runs from Khanaqin, on the Iranian border, to Baghdad's Dora refinery. At Baiji, an official of Iraq's North Oil Company said the Salahidin refinery had shut down because it had reached its storage capacity and pipeline sabotage was stopping it transporting its products further afield. Baghdad and other cities are in the grip of grave shortages of petrol and heating oil as well as gas for cooking. It is partly a result of attack on pipelines and due to attacks on convoys that import much of Iraq's energy. Though it possesses oil reserves second only to those of Saudi Arabia, Iraq has little capacity to refine energy products as a result of years of sanctions, war and now attacks. **PHOTO CAPTION*** The casket of New York City firefighter, and member of the U.S. Army Reserve 69th Infantry, Christian Engeldrum, is taken from St. Benedict's Roman Catholic Church in the Bronx while his family looks on after his funeral in New York December 9, 2004. (REUTERS)