IRBIL, Iraq -- In nearly simultaneous strikes yesterday, 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.

The blasts killed senior members of the two 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 KDP and PUK headquarters came on the first day of the celebration of Eid al-Adha, the festival commemorating the Quran's account of Abraham's willingness to sacrifice his son Ishmael at God's command.

The blasts were the first major strikes in Iraq in which the assailants employed means similar to those used against Israelis by 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.

"It was an attack by terrorists, al-Qaida and Ansar al-Islam," said Iraqi Foreign Minister Hoshyar Zubari, who is also a member of the KDP.

Iraqi and U.S. officials have long suspected Ansar, a Kurdish Islamic fundamentalist organization, of providing logistical support for foreign fighters and Iraqi insurgents. They have suggested the group has ties to al-Qaida and has particular expertise in suicide bombings.

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 yesterday 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.

Yesterday's bombings of Kurdish sites were precisely 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. One U.S. military civil affairs officer had just left the KDP building when that blast went off, Kurdish officials said.

At the green and cream PUK building, the blast destroyed a wall, charred the interior and left a jumble of furniture and clothing. Bits of flesh littered the scene. At the sand-colored KDP office, a mix of blood, chairs, tables, body parts and burned clothing made for a hellish scene. A clock blown off the wall was stopped at 10:30 a.m.

The gatherings at both buildings were held under lax security. Guards said they were under orders not to search participants as they entered.

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 yesterday.

Militants used the occasion of last fall's holy month of Ramadan to launch a devastating series of car bombings, mostly in Baghdad. On Saturday, a car bomb destroyed the facade of a police station in Mosul, 45 miles west of Irbil, and killed nine Iraqis.

U.S. military officials have expressed frustration in trying to stem an insurgency whose organization is shadowy and whose leadership is unknown. Repeated campaigns by U.S. forces against insurgents have been unable to halt or significantly reduce attacks. U.S. forces are constantly ambushed and threatened by the prospect of roadside bombs. The guerrillas regularly target Iraqi civilian and police officials with virtual impunity.

At the same time, the Bush administration is in the process of reducing the number of troops in Iraq from 130,000 to 105,000. U.S. officials say the reduction will not hamper U.S. operations, but they have also not suggested that replacement troops will improve the situation.

Iraqi Kurdistan has been the safest part of the country since the fall of Saddam. During the previous dozen years, it had enjoyed independence from Baghdad. U.S. and British warplanes protected this stretch of the north against incursions by Saddam's troops.

The PUK and KDP are former rivals who have found common cause in a drive for Kurdish autonomy in the north, but their desire to include the oil-rich region of Kirkuk in an expanded zone has been widely opposed. The KDP rules the western half of the current zone and the PUK governs the east.