Violence in Baghdad and central Iraq on Monday also killed 20 Iraqis and wounded 40 others.
The killing of five soldiers marked the deadliest day for U.S. forces in Iraq in more than two years, according to Al Jazeera. In its statement announcing the deaths, the U.S. military gave no further details about how or where exactly the soldiers were killed.
However, according to Xinhua, an Iraqi security source told reporters that at least three Katyusha rockets were fired at dawn against a joint Iraqi-U. S. military base in Baladiyat district, eastern Baghdad, close to Sadr City district, the main stronghold of Mahdi Army militia that is loyal to radical Shiite cleric of Moqtada al-Sadr.
The base is also a former site of internal security directorate under Saddam Hussein's regime.
"This morning, the American base at Loyalty Camp came under rocket attack. There was a lot of smoke inside and the Americans died in that attack in the Baladiyat area," a senior Iraqi security official said, Reuters reports.
U.S. forces officially ended combat operations in Iraq last August ahead of a scheduled U.S. troop withdrawal at the end of this year. American troops are now mainly involved in a support and training role, and helping Iraqi security forces in counter-terrorism operations.
U.S. fatalities in Iraq since last year have become more sporadic. Two U.S. soldiers were killed in May.