BAGHDAD, Feb 7 (Reuters) - A civilian was killed in a U.S. air strike on Baghdad on Thursday after Iraqi neighbourhood police mistakenly opened fire on a U.S. boat patrol, the U.S. military said.

The incident, the latest in which innocent Iraqis have been killed, occurred after neighbourhood police, called "concerned local citizens" (CLC) by the U.S. military, shot at a U.S. forces on patrol on the Tigris River south of the capital.

The U.S. soldiers returned fire in self-defence and called up support from the air. A building next to the CLC checkpoint was hit, killing the civilian.

"Coalition forces sincerely regret when any innocent civilian is killed during our operations," U.S. military spokesman Major Winfield Danielson said in a statement.

CLC units are mainly formed of Sunni Arabs, many of whom had fought in the insurgency against U.S. and Iraqi forces but turned against al Qaeda last year because of its indiscriminate killings and harsh interpretation of Islam.

They are credited as a key factor in the 60 percent fall in attacks across Iraq since last June.

"We only engage what we believe to be hostile threats and take every precaution to protect innocent civilians, specifically to minimise the chances of a tragedy like this," Danielson said.

The killing of Iraqi civilians by U.S. soldiers has long put a strain on relations between Baghdad and Washington.

Critics say U.S. forces often fire on militants without taking reasonable care to find out who else is in the area. The U.S. military says militants often deliberately use civilians as shields against attacking forces.

On Monday, the U.S. military admitted its soldiers had accidentally killed nine Iraqi civilians, including a child, in a strike south of Baghdad.

The following day, it said an innocent women had been killed during a raid by U.S. soldiers on a house holding suspected al Qaeda militants in a small town of Dour near Tikrit, 175 km (110 miles) north of Baghdad.

That incident is being investigated. So far it has not been possible to determine who killed her during a gunbattle between men firing from inside the house and U.S. soldiers outside.

The U.S. military said that when soldiers entered the house a man was found holding a woman as a human shield and pointing a rifle. He and another armed man in the house were killed along with a different woman and a young girl was wounded. (Reporting by Michael Holden; Editing by Richard Balmforth)

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