BAGHDAD — U.S. forces battled Iraqi police and gunmen today, killing six policemen, after an American raid captured an Iraqi police lieutenant accused of leading a cell of Shiite militiamen, the military said.

Seven gunmen also died in the fight, a rare open street battle between American troops and policemen. Washington has demanded the government purge its police force of militants, and U.S. and Iraqi authorities have arrested officers in the past for militia links. But the Bush administration said in an assessment Thursday that progress on that front was "unsatisfactory."

The lieutenant was captured before dawn in eastern Baghdad, but the soldiers came under "heavy and accurate fire" from a nearby Iraqi police checkpoint, as well as intense fire from rooftops and a church, the military said in a statement.

As the Americans fired back, U.S. warplanes struck in front of the police position, without hitting it directly, "to prevent further escalation" of the battle, it said. There were no casualties among the U.S. troops, but seven gunmen and six of the policemen firing on the Americans were killed, the statement said.

The captured lieutenant was a "high-ranking" leader of a cell suspected of helping coordinate Iranian support for Shiite extremists in Iraq as well as carrying out roadside bombings against mortar attacks on U.S. and Iraqi forces, the military said.