In nearly simultaneous strikes Sunday, a pair of suicide bombers set off explosives during Muslim holiday celebrations inside two buildings housing offices of the main Kurdish parties in northern Iraq, killing at least 56 people and wounding more than 200.
Elsewhere, an American soldier was killed and 12 were wounded in a rocket attack on a logistics base in Balad, 50 miles north of Baghdad, the U.S. command said. Two soldiers were in serious condition, six in stable condition and four soldiers were treated for superficial wounds.
Another soldier was killed Sunday and two others hurt when their Humvee overturned near the town of Haditha.
The deaths raised to 524 the number of U.S. service members who have died since the Iraq conflict began in March.
The attack in Irbil, 200 miles north of Baghdad, was believed to be the deadliest since an Aug. 29 car bombing in the Shiite holy city of Najaf killed Ayatollah Mohammed Baqir al-Hakim and more than 100 others as they emerged from Friday prayers. There have been several suicide car bombings in Iraq in recent weeks, and authorities are concerned they may be the work of al-Qaida.
The explosions killed senior members of the two main Kurdish parties, the Kurdistan Democratic Party and the Patriotic Union of Kurdistan, which are among the best-organized and staunchest U.S. allies in Iraq. Both groups supported the invasion that toppled President Saddam Hussein and put their large militias at the service of U.S. commanders.
The attacks in the crowded auditoriums at the parties' headquarters came on the first day of the celebration of Eid al-Adha, or Feast of Sacrifice, which commemorates the Koran's account of the prophet Abraham's willingness to sacrifice his son at God's command.
The blasts were the first major strikes in Iraq in which the assailants wore explosives, as do Palestinian suicide bombers. Most suicide bombings in Iraq have involved explosives packed into cars or trucks.
The dead included Sami Abdul Rahman, deputy prime minister of the Kurdish north. The KDP regional director for Irbil, the city's mayor and his deputy, and the chief of police also died. The PUK dead included the top representative for Irbil.
There was no immediate claim of responsibility for the bombings. But several prominent Kurds, including Iraqi Foreign Minister Hoshyar Zebari, pointed the finger at Ansar al-Islam, an al-Qaida-linked radical Islamic militia made up mostly of native Kurds. Zebari is a member of the KDP.
Iraqi and U.S. officials have long suspected Ansar of providing logistical support for foreign fighters and Iraqi insurgents. They have suggested the group has expertise in suicide bombings.
Sunday's bombings were timed to detonate at 10:30 a.m., when large crowds gathered at both headquarters and pressed into the small auditoriums as they wound in a large line to greet the dignitaries.
The bomb at the KDP headquarters exploded first, followed moments later by the explosion at the PUK building.
The gatherings at both buildings were held under lax security. Guards said they were under orders not to search participants as they entered.
"It was considered too embarrassing to search people at this happy occasion," said Ahmed Ali Ahmed, a guard for the KDP. "I myself complained about it, but those were the orders."
"There was probably too much goodwill shown by both PUK and KDP security forces," said Qubad Talabani, a PUK spokesman in Baghdad.
The lax security was all the more striking because, with the arrival of the Eid holiday, Iraq has been in a state of high alert. U.S. officials had warned of an upsurge in violence during the four-day holiday that began Sunday.
U.S. administrator L. Paul Bremer pledged to work with Iraqi security forces to capture those behind Sunday's bombings. The attackers "are seeking to halt Iraq's progress on the path to sovereignty and democracy," Bremer said in a statement.
Also Sunday, about six Iraqis were killed when they accidentally set off an explosion while looting a former Iraqi munitions dump in the desert 112 miles southwest of the southern city of Karbala, said a spokesman for Polish forces.
_ Information from the Washington Post, Associated Press and Cox News Service was used in this report.