Baghdad, Iraq ? At least 20 American service members were killed in military operations Saturday in the deadliest day for U.S. forces in two years, including 13 who died in a helicopter crash and five slain in an attack by militia fighters in the holy city of Karbala, military officials said.

Saturday’s toll was the third-highest of any single 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. U.S. authorities also announced two American combat deaths from Friday.

The heavy toll comes at a critical time of rising congressional opposition to President Bush’s decision to dispatch 21,500 additional soldiers to the conflict to try to curb sectarian slaughter.

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

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.

Five U.S. soldiers were killed Saturday night when militia fighters attacked 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.

Brooks said Iraqi officials and security forces as well as U.S. troops were present at the meeting, but his statement did not mention other casualties from the attack. It said the headquarters had “been secured by coalition and Iraqi security forces.”

Earlier, Karbala Gov. Akeel al-Khazaali had reported that U.S. troops raided the provincial headquarters looking for wanted men but left with no prisoners. But Brooks said that report was incorrect.

The general did not identify any group suspected of staging the attack, but residents reached by telephone had reported seeing military helicopters flying over the local headquarters of radical Shiite cleric Muqtada al-Sadr.

Al-Sadr’s Mahdi Army militia, which has been accused of playing a big role in sectarian killings, has been hit repeatedly in recent weeks by operations in which key commanders have been captured or killed by U.S. and Iraqi troops.

Also Saturday, roadside bombs killed a soldier in the capital and one in Nineveh province north of Baghdad.

The U.S. military also announced that combat Friday had killed an Army soldier in Nineveh province and a Marine in Anbar province, a Sunni insurgent stronghold west of the capital. The Marines often delay death reports, raising the possibility that Friday’s toll was higher.

The helicopter crash was the fourth deadliest since the start of the war. The worst crash occurred on the war’s deadliest day, Jan. 26, 2005, when a Marine transport helicopter crashed during a sandstorm in Iraq’s western desert, killing 30 Marines and a sailor. On the same day, six other U.S. forces died in combat for a total of 37 deaths.

The second highest daily toll was on March 23, 2003 when 28 service members were killed as American forces were pushing toward Baghdad on the third day of the U.S.-led invasion.

Meanwhile, the first reinforcements of U.S. troops under the new Bush strategy have already started to flow into the Baghdad region. A brigade of the U.S. 82nd Airborne Division, part of the buildup, has arrived in Baghdad and will be ready to join the fresh drive to quell sectarian violence in the capital by the first of the month, the American military said today.

The 2nd Brigade of the 82nd Airborne consists of about 3,200 soldiers who will “assist Iraqi Security Forces to clear, control and retain key areas of the capital city in order to reduce violence and to set the conditions for a transition to full Iraqi control of security in the city,” the military said in a statement.

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.

Elsewhere in Baghdad, Iraqi police and hospital officials said a joint U.S.-Iraqi force searched a hospital in the volatile Sunni-dominated western neighborhood of Yarmouk. The raid occurred at 4:30 a.m.