BAGHDAD, May 14 (Reuters) - Clashes between security forces and Shi'ite gunmen killed six people and wounded 28 in Baghdad overnight, Iraqi police said on Wednesday.

The clashes put further strain on a weekend agreement to halt nearly two months of fighting in the capital.

Police said gunmen fought U.S. and Iraqi forces in the east Baghdad stronghold of anti-American Shi'ite cleric Moqtada al-Sadr. Five people were killed and 22 wounded overnight, police said, without giving details on the casualties.

Fighting also erupted in western Baghdad's Shula district, another bastion of Sadr's Mehdi Army militia.

On Wednesday, heavy automatic weapon fire echoed through the streets of Shula as U.S. Apache attack helicopters hovered overhead. Shops were closed and residents stayed home.

Police said one person had been killed and six wounded in the Shula fighting, which began on Tuesday.

A spokesman for U.S. forces in Baghdad, Lieutenant-Colonel Steven Stover, earlier said Tuesday had been quieter in Sadr City compared to previous days. He could not be immediately reached for comment on the latest fighting.

Iraq's ruling Shi'ite alliance and Sadr's opposition movement in parliament reached an agreement on Saturday to end fighting in Sadr City that has killed hundreds of people.

A senior political aide to Sadr has urged patience with the truce, saying it might take time to filter down.

It is not clear how much control Sadr has over some of the tens of thousands of gunmen who profess allegiance to him.

Fighting flared in late March when Prime Minister Nuri al-Maliki ordered a crackdown against Shi'ite militias in Baghdad and the southern city of Basra. (Reporting by Waleed Ibrahim and Tim Cocks, editing by Keith Weir)

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