BAGHDAD - A car bomb explosion tore through the United Nations headquarters in Baghdad on Tuesday, destroying part of the building, and witnesses said at least three people were killed and dozens wounded.

"We can confirm that there was a car bombing at the Canal Hotel at 4:30 p.m. (1230 GMT)," a U.S. Army spokeswoman said.

Two casualties were taken away by helicopter and two others by truck, Reuters correspondents on the scene said. Sirens wailed and a column of thick black smoke billowed into the sky.

U.S. soldiers told journalists to keep back, warning of the possibility of secondary explosions. "A car exploded. I was standing at the gate when it exploded," said Uday Ahad, an Iraqi security guard at the building. "Many Iraqis and foreigners were wounded."

He said he had carried three dead from the rubble.

Hundreds of people work at the United Nations compound at the Canal Hotel building in eastern Baghdad.

"Suddenly there was an explosion and everything fell down. There are lots of people inside because no one had gone home yet," said Fouad Victor, a UN employee who was inside the building when it was hit by the blast.

The explosion came just days after a truck bomb blew up outside the Jordanian embassy in Baghdad, killing 17 people.

The Canal Hotel, a sprawling compound, on the western edge of the city centre, was attacked by looters during the fall of Baghdad to U.S. forces.

The hotel complex was the base for UN weapons inspectors during the years of hunting for banned biological, chemical and nuclear weapons under now deposed leader Saddam Hussein.

It also houses offices of other UN agencies.

The United Nations is playing a limited role in postwar Iraq, with the U.S.-led invasion forces assuming military and civilian control of the country.

After sharp splits with allies on the UN Security Council over the war, Washington has shown little haste in seeking a bigger part for the United Nations in Iraq, although it would like more countries to share the burden of running the country.

Taha Yassin Ramadan, former vice president of Iraq and one of Saddam Hussein's closest allies, was in U.S. custody on Tuesday after being captured by Kurdish fighters and handed over to American troops.

The Patriotic Union of Kurdistan said it had tracked down Ramadan in Mosul -- the same city Saddam's feared sons Uday and Qusay were hiding in when U.S. troops killed them last month.

The PUK is one of two Kurdish groups in northern Iraq whose forces fought alongside American soldiers in the war to topple Saddam.

The Pentagon confirmed that Ramadan, number 20 on a U.S. list of the 55 most-wanted Iraqis, had been captured. Washington says 38 of the people on the list have now been captured or killed. But Saddam himself remains on the run.

Ramadan, one of the most hawkish members of Saddam's inner circle, has been accused of crimes against humanity for his role in suppressing a Kurdish rebellion in northern Iraq in the 1980s and a Shiite revolt in the south in 1991.

The U.S. military says supporters of Saddam, and some foreign militants, are behind a guerrilla campaign that has killed 61 U.S. soldiers since the start of May.

In the latest ambush, a rocket-propelled grenade and gun attack on a U.S. convoy north of Baghdad wounded two American soldiers early on Tuesday, the U.S. military said.

Lieutenant Colonel William MacDonald of the 4th Infantry Division said the convoy was attacked near Balad, a town in the "Sunni Triangle" where support for Saddam remains strong. AMMUNITION DUMP EXPLODES

At least seven Iraqis were killed on Monday in a blast at an ammunition dump north of Saddam's home town of Tikrit. MacDonald said U.S. troops investigating the blast found one body at the scene and Iraqi police later found a further six.

The identities of those killed were not known, but soldiers said the ammunition dump had been often targeted by looters.

In Ramadi, another Sunni stronghold west of Baghdad, witnesses said a U.S. convoy was attacked with rocket-propelled grenades on Tuesday morning and at least one soldier was wounded. A U.S. Army spokesman had no immediate information.

In Tikrit, where the 4th Infantry Division has mounted scores of raids hunting Saddam loyalists blamed for the guerrilla attacks, U.S. soldiers opened fire on an ambulance during a battle with gunmen in another vehicle.

"Last night we had a vehicle with armed men apparently using an ambulance to mask its movements after curfew," said Lieutenant Colonel Steve Russell. "Reports from Iraqis and also our soldiers said the vehicle engaged our soldiers from the vicinity of the ambulance. Our soldiers returned fire resulting in the wounding of an Iraqi male in the ambulance and the slight wounding of a second man."

Human rights groups say Iraqi civilians have been killed or wounded by U.S. soldiers in several incidents, either mistakenly shot or caught in the crossfire during battles.

On Sunday, award-winning Reuters cameraman Mazen Dana, a Palestinian, was shot dead by a U.S. soldier while filming outside a prison on the western outskirts of Baghdad. The U.S. military said the shooting was a "tragic accident".

In northern Iraq, engineers worked to repair the main oil export pipeline to Turkey, a crucial lifeline for the floundering economy. The pipeline reopened last Wednesday for the first time since the war, but was shut down just two days later after an attack by saboteurs set it ablaze.

Paul Bremer, the U.S. governor of Iraq, said on Monday that sabotage had cost the economy "literally billions of dollars".