BAGHDAD, Iraq - A U.S. Army helicopter crashed northeast of the capital Saturday, killing all 13 people on board.

American and Iraqi troops stepped up raids on Sunni and Shiite gunmen, foreshadowing tactics expected to define the drive to tame Iraq's sectarian bloodshed.

The military also reported that five American soldiers were killed in a militia attack in Karbala.

The military gave little information on the Black Hawk's crash during good weather in Diyala province, where U.S. and Iraqi forces have been battling Sunni insurgents and Shiite militias around the city of Baqouba for months.

Lt. Col. Josslyn Aberle, a U.S. spokeswoman, said the cause of the crash had not been determined. Navy Capt. Frank Pascual, a member of a U.S. media relations team in the United Arab Emirates, told Al-Arabiya television that the helicopter was believed to have suffered technical troubles before going down.

It was the fourth deadliest crash since the war started in March 2003. The worst occurred Jan. 26, 2005, when a Marine transport helicopter went down during a sandstorm in the western desert. Thirty Marines and one sailor were killed ¡ª the most U.S. personnel to die in a single incident in Iraq.

Also Saturday, the military announced the deaths of two U.S. soldiers and a Marine. One soldier was killed Saturday in northern Baghdad, the other a day earlier in Nineveh province in the north. The Marine was killed Friday in Anbar province, the insurgent stronghold west of the capital.

The crash and other deaths come at a critical time for U.S. forces, as the first of 21,500 reinforcements are arriving in Baghdad and surrounding areas to join a campaign that President Bush and Iraqi Prime Minister Nouri al-Maliki are waging to curb sectarian slaughter.

In south Baghdad, U.S. helicopters dropped Iraqi police commandos into the dangerous Dora neighborhood to stage a raid on the Omar Brigade, an al-Qaida-linked Sunni militant group, Interior Ministry spokesman Abdul-Karim Khalaf said.

Khalaf said 15 insurgents were killed and five captured during an intense battle at two abandoned houses taken over by Sunni gunmen, who he blamed for a series of kidnappings and killings in a bid to cleanse the once-mixed neighborhood of Shiite residents.

"We were provided with helicopter support by our friends in the multinational forces and we did not suffer any casualties," Khalaf said. U.S. aircraft gave covering fire, but the U.S. military did not respond to a request for comment on the raid.

In the Shiite holy city of Karbala, U.S. troops raided the headquarters of the provincial government looking for wanted gunmen but left with no prisoners, Gov. Akeel al-Khazaali said. The Americans used stun grenades, which led neighbors to report the building was under mortar attack, he said.

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