The move, a dramatic reversal of US policy, came as General John Abizaid, the new coalition commander, said that tours of war-weary US troops are to be extended to at least a year to combat an increasingly organised and lethal guerilla enemy.

Already under pressure over the war’s rising cost and death toll, the Administration was dealt another significant blow yesterday after a leading Republican announced a Senate investigation of the White House’s role in President Bush’s controversial State of the Union speech in January.

In devastating testimony before the Senate Intelligence Committee on Wednesday night, George Tenet, the CIA director, said that White House officials insisted that claims about Iraq’s alleged nuclear programme be inserted in the speech, despite CIA reservations.

Mr Tenet, forced by the White House to take the blame for a now discredited claim in the speech that Iraq tried to buy uranium from Africa, “told us who the (White House) person was”, Senator Dick Durbin, a Democrat member of the panel, said.

“The person was insistent on putting this language in, which the CIA knew to be incredible,” Mr Durbin, who opposed the war, said. “All roads still lead back to 1600 Pennsylvania Avenue (the White House).”

Pat Roberts, the committee’s Republican chairman, announced public hearings into pre-war intelligence for September and said that White House officials may now be called before the panel.

“I think there were mistakes made all the way up the chain,” Mr Roberts said. He said that the committee would “follow the trail wherever it leads”, adding: “We’ll let the chips fall where they may.”

Saxby Chambliss, another Republican on the Senate panel, said that if it was necessary to question Donald Rumsfeld, the Defence Secretary, and Condoleezza Rice, Mr Bush’s National Security Adviser, “then we will”.

Jay Rockefeller, the leading Democrat on the committee, said that the panel was going to investigate whether the claim in the speech “was part of a pattern of misleading by the Administration”. He said: “I think others in the Administration knew about it.”

The FBI also announced its own investigation yesterday into how bogus intelligence made its way into the President’s speech. That investigation will focus on how forged documents, alleging that Iraq sought uranium from Niger, made their way into US intelligence agencies.

Colin Powell, the Secretary of State, said that he had started discussions with other countries on a possible new UN mandate for Iraq, which would significantly broaden UN authority over the administration of the country.

Until now, the US has sought to limit UN activities in Iraq to humanitarian relief and has sought military assistance from countries on an ad hoc basis.

That approach has appeared increasingly futile, particularly after India, Pakistan and France all announced their refusal this week to send troops to Iraq without a UN mandate. The pressure on US troops caused by increasingly deadly attacks has become so great that Pentagon plans to send up to 10,000 part-time National Guard soldiers to Iraq were revealed yesterday.

Reflecting Washington’s increasing desperation for additional troops, as morale among US soldiers drops and public discontent grows over the death toll, General Powell said: “There are some nations who have expressed the desire for more of a mandate from the United Nations and I am in conversations with some ministers about this.”

Kofi Annan, the UN Secretary-General, said: “The question has been posed as to whether or not the Security Council may not help to improve the situation — a Security Council action that expands UN activities, and perhaps appeals to member states to make troops, policemen and other resources available for the stabilisation of Iraq.”

Diplomats said that the UN discussions would focus on the idea of a new resolution similar to the one that established the International Security Assistance Force for Afghanistan, based in Kabul.

The Iraqi force would be international in make-up and authorised by the UN, but is likely to remain under overall US leadership, diplomats said.

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