It was the deadliest day for American troops in Iraq in more than two years and comes months before most U.S. soldiers are scheduled to withdraw.
American authorities did not release details of the U.S. deaths. But Iraqi officials told Western media that the incident happened in Baghdad when rockets hit a U.S. installation.
Iraqi officials also said a suicide attacker detonated a bomb-laden car in the city of Tikrit, killing at least 13 Iraqis and wounding more than 15 others.
The blast happened Monday outside a compound that houses several palaces built during the rule of the late Iraqi leader Saddam Hussein. Officials say an Iraqi army colonel and several soldiers are among the dead.
The attack follows a series of coordinated bombings in Tikrit last Friday that killed 21 people.
Tikrit was Saddam’s hometown and is located about 130 kilometers north of the capital, Baghdad.
Some information for this report was provided by AP and AFP.