Lieberman was a strong backer of the war on Iraq but has become the latest Democrat heavyweight to accuse President George W. Bush of misleading the US public over the Iraqi threat.

Lieberman released a statement saying the "troubling reports" need "full and thorough investigation."

The administration has been on the defensive for several days over reports that the Central Intelligence Agency warned the White House last year that documents saying Iraq had tried to buy enriched uranium from Niger were false.

Bush raised these allegations in his State of the Union speech to the nation in January, much of which was devoted to the need for a tough line on Iraq.

"We now know that the information in the State of the Union was false and misled the American people," said Lieberman.

"This breaks the basic bond of trust we must have with our leaders in times of war and terrorism."

He added: "Quite simply, we need to know what people in the administration knew about the weakness of our uranium intelligence reports and when they knew it."

News reports have said that the CIA warned the US and British authorities in late 2002 that there were doubts about the Niger uranium documents.

Bush, who is on a tour of Africa, said Friday US intelligence agencies had cleared his State of the Union address.

"I gave a speech to the nation that was cleared by intelligence services. It was a speech that detailed to the American people the dangers posed by the Saddam Hussein regime," Bush said in Uganda.

"My government took the appropriate response to the dangers, and as a result the world is more secure and more peaceful."

Senator John Kerry, another leading Democrat presidential candidate who voted in favour last year of authorising Bush to order military action against Saddam Hussein, on Thursday accused the Bush administration of arrogance in its post-war Iraq policy by not creating an international force to secure the country.

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