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 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 Qaeda 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.
April has been the deadliest month for U.S. forces since September, when 25 American troops 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.
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 371 Iraqis killed - in addition to 80 Iranian pilgrims - in violence in April. In March, 335 people were killed in violence in Iraq; 288 in February and 242 in January, according to the AP figures.
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" but stressed the violence remained low compared to previous years.
Tensions rose Friday in northern Iraq after a pre-dawn raid that killed two brothers in Saddam Hussein's hometown of Tikrit.
The provincial governor and tribal leaders blamed the U.S. military and demanded an apology, saying the raid violated a security agreement regulating U.S. forces' conduct.
The U.S. military, however, said the operation was conducted by Iraqis backed by a small number of U.S. advisers to arrest a wanted suspect.
"This was a combined Iraqi-Coalition Forces mission, based on evidence and a warrant," said Maj. Derrick Cheng, a spokesman for U.S. forces in northern Iraq. He said two men were killed but did not provide identities.
Security was tightened in Tikrit after hundreds of angry mourners - some firing weapons into the air - took to the streets to protest the killings.
The demonstrators shouted "Down with America!" and promised revenge.
The action came nearly a week after anger broke out over a deadly U.S. raid in the southern Shiite city of Kut. Prime Minister Nouri al-Maliki called that raid a "crime" that violated the security pact.
Iraqi police, meanwhile, said they had foiled a planned suicide bombing and arrested the suspected attacker near a Shiite mosque in the northern city of Kirkuk.
Panicked worshippers fled after hearing of the attempt, and some were wounded by broken glass as they tried to jump through mosque windows to escape.