Three U.S. 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 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 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 and letting Iraqis take the lead in security operations. 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.

April has been the deadliest month since September, when 25 American forces died.

The U.S. military reported 17 Americans killed in February and 16 in January.

April also saw the most troops killed in combat so far this year, as opposed to other causes. Thirteen of last month's 18 deaths were in combat compared with four among the nine in March.

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.

The military statement did not provide further details about the nature of the fighting in Anbar. It also did not identify the troops, pending notification of relatives.

Civilian deaths in Iraq in April were also higher than previous months following a series of bombings that killed more than 200 people.

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.

That compares with an Associated Press tally of at least 365 Iraqis killed — in addition to 80 Iranian pilgrims — in violence in April. In March, 335 people were killed in violence in Iraq; 283 in February and 242 in January, according to the AP figures.

President Barack Obama, who has ordered U.S. combat operations to withdraw from Iraq by the end of August 2010, called the recent attacks a "legitimate cause of concern" but stressed the violence remained low compared to previous years.

"You haven't seen the kinds of huge spikes that you were seeing for a time. The political system is holding and functioning in Iraq," he said Wednesday in Washington.