BAGHDAD, Sept 7 (Reuters) - Iraqi soldiers have captured a suspected Shi'ite militant commander who led a group of more than 750 fighters in southwestern Baghdad, the U.S. military said on Friday.

The military said the group -- a large force to be commanded by one man -- carried out roadside bomb attacks and sniper attacks against U.S. forces and civilian contractors in the Shi'ite district of Amil.

The "alleged brigade commander" was detained on Thursday in Baghdad along with another Shi'ite militant, the military said.

"Intelligence reports indicate the suspects are cooperating with foreign fighters and assisting other insurgents in acquiring sniper training in Iran," the military said in a statement.

Asked to confirm the number of 750 was correct, a U.S. military spokesman said the figure was based on intelligence reports. Most Shi'ite militants are believed to operate in small cells.

U.S. and Iraqi forces have stepped up raids against Shi'ite militant cells in Baghdad as part of a crackdown on sectarian violence that has killed tens of thousands of Iraqis.

Earlier this week the U.S. military said soldiers detained a "highly sought individual" suspected of links to senior officers in Iran's Revolutionary Guards.

U.S. commanders have repeatedly accused Iran's Revolutionary Guards force of training Shi'ite militias in Iraq and supplying them with increasingly sophisticated weaponry to kill American soldiers. Shi'ite Iran denies the charges.

The U.S. military statement said the two detainees caught on Thursday were believed to have ties to an "execution committee that is responsible for assassinating innocent civilians fighting for a safe and secure Iraq".

In a separate operation, Iraqi special forces detained four suspected Shi'ite militants in Baghdad on Thursday, the statement added.

One of the detainees was reported to be a physician who worked in a public clinic where he allegedly killed four other doctors, the statement said. It gave no other details on the physician.

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