U.S. forces backed by Iraqi troops surged into the heart of Falluja on Tuesday, taking a grip on Iraq's most rebellious city after a day of intense street-to-street combat. U.S. tanks and armoured personnel carriers operating in the northern part of the city came under fierce assault from rebels firing rocket-propelled grenades and AK-47 rifles after sunset, but in other areas large-scale fighting died down after dark. There was the occasional blast of mortar fire, and U.S. war planes targeted some buildings, but residents and a Reuters correspondent said heavy bombardments and explosions had eased. Some U.S. tanks were seen pulling back from central areas of the city for the night. Others remained in place. An American soldier wounded in Falluja said he had seen two of his colleagues killed. "A buddy of mine and another soldier were killed and I have seen about 50 other wounded (U.S.) soldiers since the fighting began," he told Reuters while awaiting medical evacuation. He declined to give his name. The Reuters correspondent saw about five wounded soldiers being flown out by helicopter and a U.S. military ambulance driver also said he had witnessed many casualties. --More 2112 Local Time 1812 GMT