Intense house-to-house fighting between insurgents and Iraqi police north of Baghdad killed 43 people, including 24 officers, the U.S. military said Friday. U.S. troops later joined the fight, aiding in a counterattack that left 18 insurgents dead, the military said.

An unknown number of "anti-Iraqi forces" ambushed a police unit based in Baqouba, 60 kilometers (35 miles) northeast of Baghdad at about 6:30 a.m. (0330 GMT) on Thursday the military said, using its standard term for Sunni insurgents.

Police fought back and U.S. troops nearby were diverted from another mission, assisted by air cover. One Iraqi civilian was also killed, eight insurgents wounded, and 27 others captured, the military said.

The attack marked some of the heaviest fighting in recent days between insurgents and Iraqi security forces, who U.S. commanders have been pressing to take over more responsibility for security, thereby allowing them to begin contemplating U.S. troop withdrawls.

With rising U.S. casualties adding to growing anti-war sentiment, U.S. leaders are eager to show that the Iraqi forces are rising to the challenge by controlling territory and inflicting casualties on their enemies.

Iraq's Interior Ministry, which commands the police, gave a slightly different versions of the clash and said those killed included Khan Bani Saad's police chief, Brig. Abbas Al-Ameri, and his brother.

A ministry spokesman, Brig. Abdel-Karim Khalaf, said forces moved into the area after learned of the presence of insurgents who were behind the ambush on Monday of a convoy of buses carrying police recruits in which at least 15 were killed 25 wounded.

"After we received information that these criminals had a presence ... we mobilized our forces and attacked the area," Khalf said. "We cannot tolerate this and that is the reason why we took action yesterday," he said.

Khalf denied police had been surprised and put the death toll among officers at 12, with 19 insurgents killed and 28 captured. He described the enemy fighters as hardcore remnants of

Saddam Hussein's former Baathist regime joined by "Takfiri elements" a term for Islamic radicals that include groups such as al-Qaida in Iraq.

The area around Baqouba has seen heavy fighting in recent weeks between armed Shiite and Sunni groups carrying out brutal revenge killings. A Shiite militia went on a rampage in the nearby city of Balad last week, killing scores of Sunnis and forcing others to flee their homes.

Also on Friday, four people were killed and five wounded in an attack on a van carrying Shiites returning from the funeral of a relative in the holy city of Najaf, said a spokesman for the police force in surrounding Diyala province

The gunmen drove up in two cars and sprayed the van with bullets about 20 kilometers east of Baqouba at 11:00 a.m. (0800 GMT), said the spokesman, speaking on condition of anonymity in keeping with police security guidelines.

Fearing similar bloodshed, authorities enforced a vehicle ban in the embattled Mosul on Friday following threats from Sunni gunmen who distributed leaflets at mosques on Thursday proclaiming the mixed Sunni-Kurdish city a part of an Islamic state declared earlier this month by an insurgent umbrella group, the Mujahedeen Shura Council.

While the insurgent's declaration has been viewed primarily as a propaganda move, fighters aligned with the Shura Council have been suspected in recent deadly attacks in Mosul, 360 kilometers (225 miles) northwest of Baghdad. The city is a battleground between Sunni Arabs relocated there by the former regime and members of the Kurdish minority native to the region.

American troop deaths hit their highest monthly total in a year on Thursday with the announcement of five more deaths, a Navy sailor and four Marines. All were killed Wednesday in volatile Anbar province, west of Baghdad, where Sunni insurgents have inflicted fully more than one-third of the 2,809 U.S. military deaths since the March, 2003 invasion of Iraq.

At least 96 U.S. troops have died so far this month, equaling the level for the whole of October 2005 ¡ª a factor in growing calls for U.S.

President George W. Bush to change strategy in Iraq. There have been only three months in which more U.S. forces died in Iraq: 107 in January 2005; at least 135 in April 2004, and 137 in November 2004.

U.S. officials have linked October's higher death toll to a historical spike in violence during the Muslim holy month of Ramadan, additional U.S. military vulnerability because of the security drive in Baghdad and the coming American midterm elections.

U.S. forces were continuing to search for a missing soldier, an Iraqi-born linguist abducted while visiting relatives on Monday. Additional checkpoints and troops on the streets has been credited with possibly helping reduce violence since the end of Ramadan.