BAGHDAD, Iraq -- At least 19 U.S. troops were killed in Iraq yesterday in the deadliest day for the American military in nearly two years.

The U.S. military also reported the deaths of two more troops the previous day. The surge in fatalities just days before President Bush's State of the Union address was likely to inject a note of urgency into the debate over his Iraq policy.

The day's deadliest episode was the crash of a Blackhawk helicopter northeast of Baghdad yesterday afternoon, killing all 13 U.S. soldiers aboard.

South of the capital in the Shiite city of Karbala, gunmen with grenades, mortars and assault rifles swarmed a provincial security building manned by U.S. and Iraqi forces, the military said. At least five U.S. soldiers were killed and three wounded while repelling the attack.

Three more U.S. troops and 44 Iraqis were also reported killed or found dead yesterday in politically motivated violence as Iraqi and American forces stepped up an effort to target Shiite militiamen and Sunni insurgents.

The deaths come as Pentagon planners and White House officials push forward with a plan to increase the number of U.S. soldiers in Iraq by 21,500. Mr. Bush and other supporters of the plan say it is an essential move to give the Iraqi government breathing space to crack down on violent groups and heal political wounds.

Opponents, including Democratic Party leaders in Congress and some skeptical Republicans, consider it a risky escalation that could bolster anti-American sentiment in Iraq and across the Middle East, and lead to more U.S. casualties. More than 3,000 U.S. troops have been killed and 22,000 wounded in Iraq since the American-led invasion almost four years ago.

U.S. military officials said the exact cause of yesterday's helicopter crash has yet to be determined, but Iraqi sources claimed it was shot down. A witness said he saw ground fire bring down the aircraft, and an insurgent group claimed responsibility for the attack in an Internet posting that could not be authenticated.

Iraqi officials and witnesses said the crash took place near the town of Buhruz in the region of Tarefiya, a rural Sunni enclave of canals and wheat fields about six miles south of Baqouba, the capital of Diyala province. The area is known as an insurgent stronghold. A recent joint U.S. and Iraqi military offensive focused on Diyala province.

A message posted to the Web by the Mujahedeen Army, an insurgent group that operates out of Baghdad and Diyala provinces, claimed responsibility for the attack.

"The helicopter was downed by [an] anti-aircraft [rocket] that belongs to the Mujahedeen Army in Buhruz," said the statement, posted to a Web site where insurgent claims are frequently trumpeted.

An Iraqi witness who spoke on condition of anonymity, said the helicopter was felled by ground fire.

"I'm not sure if it was a rocket or other projectile," said the man, a farmer. "After the helicopter was fired upon, it was obvious that it was losing control. Then it crashed with an explosion and the smoke started." The farmer said he and others dared not approach the wreckage to look for survivors, fearing that U.S. forces arriving on the scene might fire at them.

The U.S. military could not confirm the account. Lt. Col. Josslyn Aberle, a spokeswoman for the U.S. military in Baghdad, said that the crash took place northeast of Baghdad at around 3 p.m.

"All passengers and crew were military service members," she wrote in response to an e-mail query, adding that more information was expected Sunday. The military withheld names of the victims pending notification of family members.

An Iraqi official who spoke on condition of anonymity said U.S. forces had cordoned off a vast area of farmland and sheep pastures.

Yesterday's crash was the first since a U.S. Marine CH-53 transport chopper accidentally went down in Anbar province Dec. 11. Insurgents last shot down a helicopter on May 14, near the village of Yusufiya south of the capital.

The deadliest day for Americans service members in Iraq was Jan. 26, 2005, when 37 U.S. troops were killed, 31 of them in the accidental crash of a Chinook helicopter in Anbar province.

The Karbala firefight erupted as U.S. and Iraqi officials planned security for the Ashura festival, an annual Shiite pilgrimage that begins today. Karbala has come under the sway of Shiite militiamen loyal to radical cleric Muqtada al-Sadr, but it also is adjacent to Sunni Arab areas. A statement released by the military did not clarify whether the attackers were suspected Sunni or Shiite gunmen.

Three other Americans were reported killed in separate incidents.

A soldier died in northern Baghdad yesterday when his convoy was struck by a roadside bomb, and the U.S. military reported that two soldiers had died the previous day.

A soldier assigned to the 1st Cavalry Division near Mosul died Friday from injuries sustained from an improvised explosive device. Two soldiers were also injured in the bombing. And a Marine died of wounds he sustained in combat in Anbar province Friday, a news release said.

U.S. forces have identified Shiite militias as greatest threat to Iraq's stability. American troops raided the capital's main emergency hospital yesterday morning, seizing the weapons of security guards believed to be linked to al-Sadr's al-Mahdi army militia, hospital and police officials said.

Iraqi forces said the U.S. military seized AK-47s and machine guns ostensibly used by the guards to protect the facility, which has come under repeated attack by insurgents.

One hospital employee said the Americans scoured through the hospital seeking a specific person who allegedly belonged to the al-Mahdi army. They were not able to identify the individual, and the guards were released after being told they would face arrest if they were seen with weapons again, Iraqi officials at the hospital said.

Iraq's Health Ministry is controlled by Shiites loyal to Mr. Sadr. U.S. military commanders have complained privately that Sadr loyalists have turned the ministry into a political and fundraising tool. Sunni Arabs complain that they're denied health-care services or targeted for kidnapping and assassination upon entering state-controlled hospitals and clinics.

The 5:30 a.m. raid on Baghdad's Yarmouk Hospital followed Friday morning's arrest of Sheik Abdul-Hadi al-Darraji, an Sadr deputy and frequent talking head.

U.S. forces also detained 25 people they described as suspected terrorists in Sunni Arab enclaves north and west of Baghdad, a news release said.

Authorities in Baghdad said they discovered the bodies of 29 Iraqi civilians, all apparently victims of sectarian death squads, that had been dumped in various parts of the capital.

Mortar battles between neighboring Shiite and Sunni enclaves in northern Baghdad left at least four dead and 18 injured. Gunmen ambushed and killed a ranking Ministry of Interior official in northern Baghdad.

Near the provincial capital of Hillah, authorities discovered the bullet-riddled body of a man in his 30s.