BAGHDAD — Three more American soldiers were killed this week, the United States military announced on Thursday, bringing the combat-related deaths for United States forces in Iraq to a monthly toll not seen since 2008.

The deaths occurred Wednesday in southern Iraq. A military spokesman confirmed that the soldiers had been killed by enemy attack.

The increase in casualties has been attributed to rocket or mortar attacks on American bases by Shiite militias. American convoys have also come under increasing threat from improvised explosive devices. Militants have stepped up their attacks, the military said, in an effort to claim credit for pushing out the American forces, who are to withdraw by the end of the year.

The casualties are particularly striking given the diminishing numbers of American forces and their reduced combat role. Fewer than 50,000 troops remain, compared with more than 160,000 at the height of the war.