The violence came a day after two UN security experts arrived in Iraq to liaison with US-led authorities on any future return of its staff. The United Nations pulled international staff out of Iraq last year after two suicide bomb attacks on its Baghdad headquarters.

US officials said the Samarra bomb was an attempt to derail council elections in the town, 100 km (62 miles) north of Baghdad. Officers said four Iraqis were killed and 40 people wounded, including seven US soldiers who were slightly hurt.

They earlier said the blast was caused by a car bomb, but later said it was a device placed on the road under the vehicle.

Near the tense town of Falluja, 50 km west of the Iraqi capital, two American soldiers were killed when a roadside bomb exploded as their convoy passed, the US Army said.

Both towns lie in an area known as the "Sunni triangle", where much of the violence against US-led occupation forces and Iraqis seen as cooperating with them has taken place.

The Samarra blast, outside the courthouse and town council buildings and close to the main police headquarters, scattered wrecked cars and broken glass across a wide area.

Bob Silverman, head of governance for the US-led administration in the area, said the bomb went off shortly before councillors were due to elect representatives to the provincial council. The vote would now be delayed a few days.

"I am sure the bomb was meant to prevent the vote," he said.

Analysts have warned that insurgents would try to derail US-led efforts to hand over power to Iraqis.

The UN team in Iraq will assess the security situation amid calls from Washington for more involvement from the world body in Iraq, in particular to determine if the country is ready to hold early national elections.

Military Police officer First Lieutenant Alexis Marks was 100 metres (yards) away when the bomb exploded. "The blast knocked me off my feet," she said. "It shook all the vehicles and everyone nearby was thrown to the ground."

Since the invasion in March, 509 US soldiers have died in Iraq, at least 351 of them in action, according to the US military. On Friday evening, a US Kiowa helicopter crashed in northern Iraq, killing the two crew aboard.

The Army has said there was no indication it was shot down.

In Washington, the White House insisted the search would go on for weapons of mass destruction in Iraq, despite the resignation of the man leading the hunt.

"I don't think they existed," David Kay said on Friday after stepping down. "What everyone was talking about is stockpiles produced after the end of the last (1991) Gulf War, and I don't think there was a large-scale production programme in the '90s."

Kay's departure had been expected, but his comments will put more pressure on President George W. Bush and British Prime Minister Tony Blair over their justification for going to war.

"We remain confident that the Iraq Survey Group will uncover the truth about Saddam Hussein's regime, the regime's weapons of destruction programmes," White House spokesman Scott McClellan said. A spokesman for Blair said: "Our position is unchanged."

The renewed controversy over the reasons for going to war comes as the occupying powers scrambled to rescue a plan for handing sovereignty back to Iraqis in the face of widespread demands across the country for early elections.

Under the original US-backed plan, regional caucuses would select a transitional Iraqi assembly by end-May and this would appoint an interim government that would take over sovereignty by end-June. Full elections would follow in 2005.

But Ayatollah Ali al-Sistani, Iraq's most revered Shi'ite cleric, wants elections to be held sooner.

After scorning the United Nations for its failure to back the war on Iraq, Washington has now asked UN Secretary-General Kofi Annan to send a team to Iraq to determine whether early elections are feasible, hoping this will end the controversy.

US officials were optimistic the United Nations would send a delegation, which is separate to the security team in Iraq.