(Updates with more information)

BAGHDAD, Aug 7 (Reuters) - Three U.S. soldiers were killed when a roadside bomb hit their convoy south of Baghdad on Aug.4 the U.S. military said on Tuesday.

The deaths bring to 21 the number of U.S. soldiers killed in the first six days of August. A total of 80 were killed in the entire month of July.

The U.S. military said in a statement that the three soldiers were members of Task Force Marne, which has deployed south of Baghdad to block the flow of weapons, explosives and Shi'ite and Sunni Arab militants into the capital.

Earlier on Tuesday, the military also reported the death of a soldier in Baghdad, who was killed on Monday when his vehicle was struck by an explosively formed penetrator, a powerful bomb that U.S. officials say Iran is supplying to Shi'ite militias.

Four other soldiers were killed on Monday in Diyala province, where U.S. troops have launched a summer campaign against militants using the area as a staging ground for car bomb attacks in Baghdad.

President George W. Bush has warned that August will be a bloody month for U.S. forces in Iraq as al Qaeda tries to influence the intensifying debate in Washington over a timetable for a troop pullout.

At least 3,680 U.S. soldiers have now been killed since the U.S.-led invasion to oust Saddam Hussein in 2003.

U.S. commanders have warned of a rise in U.S. casualties as troops push into dangerous areas previously off-limits to U.S. forces. Bush has sent an extra 30,000 troops to Iraq to help stabilise the country.

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