BAGHDAD, Iraq - Insurgents set off a series of explosions today, killing 12 police officers near Kirkuk and hitting a Defense Department convoy as a senior American official visited the capital, the U.S. military and Iraqi police said. Al-Jazeera television broadcast what it said was video of a kidnapped American.

The tape showed a man sitting behind a wooden desk as three men pointed their guns toward him. He was holding what looked like a passport and a photo identification. The U.S. Embassy announced the kidnapping of an American citizen on Tuesday. The unidentified American was a contract worker abducted Monday, a spokesman said.

The U.S. military said the Baghdad attack on the U.S. convoy damaged two government SUVs and five civilian cars. It was unclear whether anyone was injured. Al-Qaida in Iraq claimed responsibility.

"A member of our martyrdom seekers' brigade mingled in an American military convoy at the airport road and exploded himself, destroying the infidels," al-Qaida in Iraq said in an Internet statement. The statement could not be independently verified.

The car bomb was among four explosions that rocked central Baghdad early today, the military said. A second car bomb did not cause any damage, and the third blast was a "secondary explosion" nearby, the military said.

The military gave no information on the fourth explosion, but twin blasts exploded near a convoy of two U.S. Humvees and a fuel tanker as it made its way through an eastern Baghdad neighborhood, witnesses at the scene said. The truck burned violently and sent up a large plume of black smoke visible across Baghdad.

Near Kirkuk, 12 policemen helping to dismantle an apparent decoy bomb were killed by another explosion today, police said. Three others were injured.

Police Brig. Sarhat Qadir said the explosion 10 miles northwest of Kirkuk occurred as a group of police were trying to cordon off the area. He said the bomb being dismantled was apparently a decoy to draw in more police before the second bomb exploded.

The violence came as Secretary of State Condoleezza Rice's top deputy, Robert Zoellick, arrived in the war-battered capital today on a one-day visit following a trip to Iraq by Defense Secretary Donald Rumsfeld on Tuesday.

His trip, like Rumsfeld's, was kept secret for security reasons until his entourage landed in Baghdad.