The U.S. military is not releasing any other information, but Iraqi security officials say they were killed in a rocket attack at an Iraqi base in eastern Baghdad. The deaths were the largest single-day loss of life for American forces in two years.
Other attacks across Iraq have left seven people dead. Four people were killed when gunmen opened fire on two security checkpoints in Baghdad.
A total of 11 people were killed in the northern city of Tikrit, the capital and near the western city of Ramadi Monday.
Four of them died when a bomb exploded at a checkpoint outside a government compound in Tikrit, the hometown of Saddam Hussein. It was the second attack in four days against the compound and the government employees who live and work there.
The deaths raised to 4,459 the number of American service members who have died in Iraq, according to an Associated Press count
There are 46,000 U.S. forces still in Iraq scheduled to depart by year's end. American troops and their bases in Baghdad and southern Iraq have increasingly come under attack and threats from Shiite Muslim militias, hoping to construct a narrative that they were responsible for driving out the Americans.
The Associated Press contributed to this story