A roadside bomb killed three American soldiers and wounded two in Baghdad.
A U.S. statement said the bomb was an explosively-formed penetrator, a type which the U.S. says Iran has been supplying to Shiite militias, a charge the Iranians deny.
The U.S. said the blast occurred Tuesday in east Baghdad but gave no further details.
However, a U.S. Humvee was seen burning Tuesday at an intersection in Mashtal section of eastern Baghdad.
Last week, anti-American cleric Muqtada al-Sadr ordered a six-month suspension of operations by his Mahdi Army militia.
U.S. officials believe mainstream Mahdi forces have generally stuck by the order but breakaway factions of the militia are continuing attacks.
Russian President Vladimir Putin and German Chancellor Angela Merkel had had a few fights and used strong language because of the Ukrainian crisis in 2014