President George W Bush has defended a controversial speech he made about the case for going to war in Iraq.
The US and UK are under pressure over their justification for war
Mr Bush said the CIA had cleared the State of the Union address he made on 28 January containing the allegation - since discredited - that Iraq had been trying to buy uranium from Niger.
In a statement later on Friday, CIA director George Tenet acknowledged his organisation had wrongly allowed Mr Bush to tell the American people that Iraq was trying to acquire nuclear material from Africa.
American media reports earlier suggested that the CIA advised the White House to remove the claims from the speech.
Amid the confusion, Mr Bush's political opponents became more vocal, with several senior Democrats calling for investigations into the affair.
Earlier this week, the White House acknowledged for the first time that the claim about Iraq seeking to buy uranium from Niger might be wrong.
Asked about this during a visit to Uganda, Mr Bush replied: "I gave a speech to the nation that was cleared by the intelligence services."
He did not answer when pressed on how the erroneous material came to be included in the address, stressing instead that his government took the right decision to invade Iraq.
Mr Bush's national security adviser Condoleezza Rice insisted the president "did not knowingly say anything that we knew to be false".
Putting the onus of responsibility on the intelligence services, she reiterated that the CIA had vetted the speech and cleared it "in its entirety".
In his January address, Mr Bush said: "The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
If anyone had any doubts about the uranium claim, "those doubts were not communicated to the president," Ms Rice told reporters.
However, she said the CIA did make some changes to that particular sentence in the speech.
"Some specifics about amount and place were taken out," she said.
"With the changes in that sentence, the speech was cleared."
The uranium claim was undermined by the International Atomic Energy Agency, which said it was based on forged documents.
It is also now clear that the state department had doubts about the allegations.
The claims were not used by US Secretary of State Colin Powell when he made his presentation to the UN about Iraq's weapons programme, eight days after the president's speech.
The BBC's Rob Watson in Washington says Democrats, previously reluctant to criticise a popular president waging what most Americans considered a just war, are now smelling political blood.
Democrat presidential candidate Howard Dean has said any senior government officials should resign if they withheld vital information.
"We need to find out what the president knew and when he knew it. This is a serious credibility issue for the United States Government," Mr Dean told American television.
Calls are also growing for a Congressional investigation into the administration's case against Iraq.
Our correspondent says most Americans remain unconcerned by such political arguments.
A potentially more serious problem for Mr Bush is the mounting financial and human cost of the war's aftermath.
The White House has now revealed operations in Iraq are costing nearly $4bn a month and with continuing American casualties in Iraq, opinion polls show plummeting public confidence in the US-led occupation.
In a related development, the Washington Post newspaper reported on Friday that the CIA tried and failed to dissuade the UK Government from including the uranium claim in an intelligence dossier published last September.
But a spokesman for the UK Prime Minister, Tony Blair, said on Friday that the British intelligence services stood by the allegations, and that Britain had separate information from that of the US.
Mr Blair is under fire from British MPs about the credibility of the dossier, which set out his case for war.