BAGHDAD, Iraq — At least 19 U.S. troops were killed in a helicopter crash and insurgent attacks across Iraq yesterday in the deadliest day for the American military here in nearly two years.

The surge in fatalities comes days before President Bush's State of the Union address and amid heated opposition to his plan to increase the number of U.S. soldiers in Iraq by 21,500.

The day's deadliest episode was the crash of a Blackhawk helicopter northeast of Baghdad, killing all 12 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.

Four more U.S. troops and 50 Iraqis were also reported killed or found dead in politically motivated violence as Iraqi and American forces stepped up an effort to target Shiite militiamen and Sunni insurgents.

The military withheld the names of the dead troops until relatives could be notified, and did not say where they had been based.

Yesterday's toll was the third-highest of any day since the war began in March 2003, eclipsed only by 37 U.S. deaths on Jan. 26, 2005, and 28 on the third day of the U.S. invasion.

Most of those killed on Jan. 26, 2005, died in the accidental crash of a helicopter. Twenty-six were Marines based at Kane'ohe Bay and one was a Navy corpsman from Pearl Harbor.

U.S. authorities yesterday also announced two American combat deaths from Friday.

Meanwhile, the first 3,200 of the additional troops ordered to Iraq by President Bush have arrived in Baghdad, the U.S. military said today in a statement. The 2nd Brigade of the 82nd Airborne Division will be "fully operational" within two weeks, the statement said.

Their mission will be to help Iraqi security forces "clear, control and retain key areas" of Baghdad "to reduce violence and to set the conditions for a transition to full Iraqi control of security in the city," the military said.

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.

The military originally said 13 soldiers died in the crash, but today corrected the number to 12.

INSURGENTS CLAIM CRASH

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 about 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 today.

Five U.S. soldiers were killed last night when militia fighters attacked a provincial headquarters in the Shiite Muslim holy city of Karbala, the military said in a statement.

The statement said "an illegally armed militia group" attacked the building with grenades, small arms and "indirect fire," which usually means mortars or rockets. The statement said three other soldiers were wounded repelling the attack.

"A meeting was taking place at the time of the attack to ensure the security of Shiite pilgrims participating in the Ashoura commemorations," said a statement from Brig. Gen. Vincent K. Brooks, deputy commander of the Multi-National Division-Baghdad.

Karbala is 50 miles south of Baghdad and thousands of Shiite pilgrims are flocking to the city to mark the 10-day Ashoura festival commemorating the death of one of Shiite Islam's most sacred saints, Imam Hussein, grandson of the Prophet Muhammad.

Also, a bomb struck a small bus in Baghdad as it headed to a predominantly Shiite area today, killing six passengers and wounding 10, police said.

The bus was en route from the Bab al-Sharqi area to the central commercial district of Karradah when the explosion occurred at 8:15 a.m., shattering the windows of nearby stores.

Meanwhile, 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.

Iraq's Health Ministry is controlled by Shiites loyal to al-Sadr. U.S. military commanders have complained privately that al-Sadr loyalists have turned the ministry into a political and fundraising tool. Sunni Arabs complain that they're denied healthcare 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 al-Sadr deputy.

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. Unidentified gunmen in another town opened fire on a crowd of people, killing one. Gunmen in the town of Madain broke into two houses and killed three people, including two women.

The Los Angeles Times and The Associated Press contributed to this report.