The U.S. military says three American soldiers were killed in southern Iraq Wednesday, in the deadliest month for U.S. service members in two years.

A statement issued Thursday gave no other details, but officials told Western news agencies that the soldiers were killed in a hostile attack. The Associated Press reports a rocket attack blamed on a Shi'ite militia linked to Iran caused the deaths.

The U.S. is withholding the names of the killed soldiers until their families have been notified.

Fifteen U.S. troops have died this month in Iraq. It is the highest number of military deaths in the country since May 2009 when American forces were still operating in Iraqi cities.

On Sunday, the U.S. military said two American service members were killed in northern Iraq while conducting operations.

Nearly 50,000 American troops are still in Iraq, but an agreement between Washington and Baghdad stipulates they will withdraw from the country entirely by the end of 2011.