The attack happened Wednesday in the southern part of the country, a few miles from the Iranian border.
The U.S. military blames the strike on a Shiite militia linked to Tehran.
The U.S. military is preparing to leave Iraq by the end of the year, as required by a 2008 security agreement between Baghdad and Washington. But as both governments now consider extending the deadline to have thousands of troops remain in Iraq into 2012 - in part to counter Iran's influence over the unstable nation - at least three major Shiite militias have stepped up attacks on soldiers to force the military out.
This has been the deadliest month for U.S. troops since June 2009.
In all, 15 U.S. troops have died this month in Iraq, all but one in hostile attacks.
The Associated Press contributed to this report.