Bombardments by suspected militants killed four US soldiers as troops tried to push Shiite fighters farther from the US-protected Green Zone and out of range of their rockets and mortars.
At least 44 American soldiers have been killed in Iraq in April, making it the deadliest month for US forces since September.
The US military said three soldiers were killed on Monday in eastern Baghdad by indirect fire, a reference to mortars or rockets. The statement did not give an exact location for the attack, but the Shiite stronghold of Sadr City has been the scene of intense fighting recently with Shiite militiamen.
A fourth US soldier was killed by a shell in western Baghdad, the military said.
Meanwhile, the US ambassador to the United Nations, speaking before the UN Security Council, accused Iran and Syria of trying to destabilise Iraq.
A showdown between the Iraqi government and the Mahdi Army - led by anti-American cleric Muqtada al-Sadr - has increasingly drawn US forces into battles. American commanders are particularly focused on trying to curb a rise in mortar and rocket attacks on the Green Zone.
At least three more salvos hit the Green Zone in central Baghdad, but there were no reports of injuries. In Sadr City, the stronghold of the Mahdi Army militia, US soldiers battled deeper into the district a day after fierce clashes that killed at least 38 suspected militants, the military said.