BAGHDAD — Three U.S. troops have been killed in fighting west of Baghdad, the military said today, making April the deadliest month for American forces in Iraq since September.

At least 18 U.S. 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 reignite 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 U.S. forces.

Two people were arrested following an anti-tank grenade attack against U.S. forces Wednesday in a mainly Sunni neighborhood in northwestern Baghdad, the military said.

The two U.S. 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.

The U.S. military did not give further details about the attack but said the Americans were providing requested support to Iraqi forces when it occurred. It did not identify the troops who were killed, pending notification of relatives.

In all, at least 4,281 members of the U.S. military have died in the Iraq war since it began in March 2003, according to an Associated Press count.

At least 355 Iraqi civilians and Iraqi security forces were killed in violence in April, according to a monthly death toll issued by various Iraqi government ministries.

President Barack Obama, who has ordered U.S. combat forces to withdraw from Iraq by the end of August 2010, called the recent attacks a "legitimate cause of concern."