It was the second car-bombing in as many days against Iraqis who cooperate with the U.S.-led occupation. Mayor Abdul Rahman Mustapha said "all indications point" to a suicide attack, because an unidentified body could be the driver.
Elsewhere, a pro-U.S. politician was assassinated in the southern port city of Basra, his party said Thursday.
President Bush and Prime Minister Tony Blair stood united Thursday on the war on terror. Blair, who has seen his approval ratings sink amid broad opposition here to the war, called the process of ensuring a stable, democratic Iraq "an essential part in defeating this fanaticism and extremism" that is killing innocents in attacks around the world.
"Our response is not to flinch or give way or concede one inch," he said. "We stand absolutely firm until this job is done."
"They need to be stopped and we will stop them," Mr. Bush said, with as many as 100,000 anti-war protesters mobilizing for a massive march on Parliament as he spoke.
In other developments:
But The Washington Post says Army leaders have warned Congress that the process of rotating tens of thousands of troops into and out of Iraq in coming months will pose serious logistical challenges. Meanwhile, the rush is on to train the Iraqi army that is supposed to replace some U.S. troops.
Jalal Johar, an official with the Patriotic Union of Kurdistan, said the powerful explosion occurred about 10:30 a.m. It shattered windows and damaged doors at the two-story, yellow-and-green PUK building and blew out the windows of the nearby radio and television station.
Security guard Assad Ahmed said he saw the pickup moving before the blast but he was unsure if the driver was inside when it exploded. Johar said security had been bolstered around the building following intelligence reports that an attack was likely.
"We think that Islamic terrorist groups and remnants of the Saddam (Hussein) regime are behind the attack," he said. "They are coordinating between them."
The PUK is a group that supports American efforts in Iraq. Party chief Jalal Talabani is the current head of the U.S.-installed Iraqi Governing Council. One of the PUK's regional rivals in the Kurdish area, Ansar al-Islam, is believed to have ties to al Qaeda. No group claimed responsibility for the attack.
Some U.S. officials suspect Ansar al-Islam is working with Saddam Hussein loyalists, including Izzat Ibrahim al-Douri, former vice chairman of the Revolutionary Command Council. U.S. officials have offered a $10 million reward for the capture of al-Douri.
U.S. troops were assisting in the investigation, said Lt. Col. William MacDonald, spokesman for the 4th Infantry Division in Tikrit.
Late Wednesday, a car bomb exploded outside the home of Sheik Amer Ali Suleiman, a tribal leader in Ramadi, about 60 miles west of Baghdad inside the "Sunni Triangle" where anti-U.S. attacks are concentrated. Hospital workers said Thursday that two people were killed.
Suleiman is a leader of the Duleim tribe, one of the largest Sunni Muslim tribes in Iraq. He also is a member of the city council and is close to the Americans.
Rebels repeatedly have attacked police stations and Iraqis perceived to be cooperating with the occupation.
In Basra, the Assyrian Democratic Movement said its representative on the municipal council, Sargoun Nanou Murado, was abducted Tuesday on his way to work. His body was found Wednesday, a statement said.
The Assyrian Democratic Movement, which represents Iraq's Assyrian minority, is represented on the 25-seat Governing Council.
The assassination is the second this week of people working with coalition authorities in southern Iraq. In the town of Diwaniyah, gunmen on Tuesday killed the education ministry's director general for that province.
On the diplomatic front, Russia said any new U.N. resolution endorsing a hand over of power to Iraqis should give a strong political role to the United Nations, and it criticized the United States for negotiating an Iraq plan in a "secretive" atmosphere.
The Iraqi council has announced a set of deadlines that would give the country a provisional national assembly by May, a transitional administration with full sovereign powers in June, and an elected government before the end of 2005.
U.N. diplomats said Monday that the United States wants a new U.N. resolution to endorse the agreement. The U.S. State Department plans to start work on drafting the resolution and close ally Britain may help with the text, the diplomats said, speaking on condition of anonymity.
Russia, Germany and France have been pressing for months for a stronger U.N. political role in Iraq.