Three US troops have been killed in fighting west of Baghdad, the military said Friday, making April the deadliest month for American forces in Iraq since September. At least 18 US soldiers died in April, a sharp increase from March's total of nine - the lowest since the war began in March 2003. The deaths come as a series of deadly bombings in recent weeks has raised concerns that insurgents are stepping up their efforts to re-ignite sectarian bloodshed and derail security gains that have brought overall violence to its lowest levels in recent years. Most of the violence has targeted Iraqis since the Americans have begun pulling back from inner-city outposts in preparation for a withdrawal from urban areas by the end of June. But attacks have continued against US forces. The two US Marines and one sailor were killed Thursday while conducting combat operations in Anbar province, according to a statement. Anbar is a former insurgent stronghold that has been relatively calm since Sunni tribal leaders turned against al-Qaida in Iraq.