Two suicide bombers with explosives wired to their bodies struck the offices of the country's two main Kurdish parties in nearly simultaneous attacks Sunday, killing at least 56 people and wounding more than 235 in the deadliest assault in Iraq in six months.
The attacks struck in the Kurdish heartland and took a heavy toll among senior leaders of Iraq's most pro-American ethnic group.
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 Irbil attackers slipped into the offices of the Kurdistan Democratic Party and the Patriotic Union of Kurdistan along with hundreds of well-wishers gathering for the Muslim holiday of Eid al-Adha, or the Feast of Sacrifice.
Kurdish television said both bombers were dressed as Muslim clerics.
Leaders of both parties, whose militias fought alongside U.S. soldiers during the invasion of Iraq last year, were receiving hundreds of visitors to mark the start of the four-day holiday when the blasts went off.
Guards said they did not search people because of the tradition of receiving guests during the holiday. Neither party's top leader - Jalal Talabani of the PUK and Massoud Barzani of the KDP - was in Irbil when the attacks occurred.
Although Iraq has suffered numerous suicide bombings in recent months, the attack Sunday marked the first time perpetrators have worn explosives rather than using vehicles.
Sunday's blasts came a day after a car bomb outside a police station in the northern city of Mosul killed at least nine people. Hours later, a mortar attack hit a Baghdad neighborhood, killing five people and wounding four.
U.S. officials said foreign militants or Ansar al-Islam, an al-Qaida-linked Islamic militant group based in the north that has frequently clashed with the Kurds, may have carried out the attacks. There was no immediate claim of responsibility.
"We have no proof at this point (about who is responsible). It could be Ansar al-Islam. It could be al-Qaida. It could be any of a number of foreign terrorist groups operating in Iraq," said U.S. Brig. Gen. Mark Kimmitt, coalition deputy chief of staff for operations.
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.
In statements, the leaders of both parties, once bitter rivals, expressed their resolve to fight terrorism together.
"These terrorist acts are against the Islamic religion and humanity and we shall work more seriously toward uniting our (Kurdish) government," Talabani said. "We will work together in order to live in a democratic, federal Iraq."
No matter who was behind them, the blasts may heighten tensions between the Kurds and Sunni Arabs. As U.S. and Iraqi leaders try to map out the country's new form of government, some Arabs have sharply opposed Kurdish demands to retain or even expand their self-rule region in the north.
Hours after the attack, a mangled head believed to be that of one bomber lay on the floor of the KDP office. Blood and bits of flesh were spattered on the walls and ceilings. The attack on the PUK office, about eight miles away, took place at about the same time.
The U.S. command in Baghdad put the casualty toll at 56 dead and more than 200 injured. Irbil city morgue director Tawana Kareem told the AP that 57 bodies were brought to the morgue and "figures are increasing." At least 235 people were admitted to the city's three hospitals with injuries, hospital officials said.
Officials said the death toll may be far higher, with some bodies buried in the rubble or taken away by relatives.
The KDP leadership took a heavy blow. Among the dead were the Irbil region's governor Akram Mintik, the deputy governor and his two sons, and the KDP Deputy Prime Minister Sami Abdul Rahman, as well as ministers in the Kurdish administration, according to Ihsan and other Kurdish officials.
The PUK's military commander also was killed, Kimmitt said.
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 a series of suicide car bombings in Iraq in recent weeks and authorities are concerned they may be the work of al-Qaida.
U.S. military officials had said they were prepared for any upsurge of violence in connection with the Eid holiday. The start of the Islamic fasting month of Ramadan last year marked a sharp escalation in violence against the U.S.-led coalition and its Iraqi allies.
Deputy Defense Secretary Paul Wolfowitz, visiting the Iraqi capital Sunday, said the bombings on the Muslim holy day showed the inhumanity of those responsible.
"They are not about Islam," he said. "They're about their own fanatical view of the world, and they will kill to try to advance it. But we're winning, and they're losing."
Under U.S.-led aerial protection, Iraq's Kurdish minority, ethnically distinct from the majority Arabs, have ruled a Switzerland-sized swath in the north of the country since the end of the Gulf War more than a decade ago.
Though they have feuded violently in the past, the KDP and PUK have worked together in recent years to run the zone, creating their own parliament in Irbil.
Kurdish leaders have been pressing for a federal system in Iraq's permanent post-Saddam Hussein government that would enshrine their autonomy. That has prompted accusations among many in the Arab majority that the Kurds seek to divide the country
Tensions have been further hiked by struggles to dominate Kirkuk, a key oil city just outside the Kurdish zone with a population equally divided among Kurds, Arabs and other ethnic groups.
The blasts could sharpen divisions between Arabs and Kurds, said Jonathan Schanzer, a terrorism expert from the Washington Institute for Near East Policy who met Kurdish officials in Irbil last week.
"I think that they (the attackers) are trying to drive a wedge between the north and the center," Schanzer said. "They will want the Kurds to circle the wagons and make them more suspicious of Arabs. This will certainly add to the fractured landscape of Iraq."
"This was an attack on all Iraqis not on any particular group," Yehia Khatib, a spokesman for the Iraqi Governing Council, said in Baghdad. "Our aim and target to build a new democratic Iraq will not be affected."
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.
Associated Press correspondents Mariam Fam in Baghdad and Louis Meixler in Ankara, Turkey, contributed to this report.