The British government proposed offering leniency to some Iraqi scientists to boost the search for weapons of mass destruction, as Prime Minister Tony Blair (search) defended himself in Parliament on Wednesday against accusations he exaggerated the threat from Saddam Hussein (search).

With the search for weapons so far coming up dry, Blair faces the biggest challenge of his six years in office: two Parliament inquiries into claims about Iraqi biological, chemical and nuclear weapons -- the main reason cited by Blair and President Bush (search) for war.

In a bid to advance the hunt, the Foreign Office said Britain proposed to the United States taking "steps that could make it easier for scientists and technicians associated with Iraq's weapons of mass destruction program to come forward with information."

The measures could include freedom from prosecution. But leniency would not be offered to those accused of war crimes or crimes committed during Saddam's regime, a Foreign Office spokesman said, speaking on condition of anonymity.

Also proposed is the creation of walk-in points, telephone hot lines, and re-employment programs for those who come forward, the spokesman said.

Donald H. Rumsfeld, the U.S. defense secretary, told a Pentagon news conference Wednesday that officials from the Justice Department and CIA are considering the British proposal to offer leniency to captured Iraqi officials in exchange for information about both weapons of mass destruction and Saddam's whereabouts.

Questions over prewar assessments of Iraqi weapons programs have been raised in London and Washington. The U.S. Congress is to begin hearings into the intelligence case for war this week, but Republicans have rejected calls for a more formal inquiry.

Blair, however, is facing a tougher time.

During a lively and aggressive question-and-answer session in the House of Commons on Wednesday, Blair insisted that Saddam had posed a threat to the Middle East and the rest of the world.

"Saddam Hussein was a threat to his region and to the wider world," Blair told legislators. "I always made it clear that the issue was not whether he was about to launch an immediate strike on Britain. The issue was whether he posed a threat to his region and to the wider world."

The prime minister has said there is not "a shred of truth" in allegations that his government manipulated evidence, and that he still believes coalition forces will find proof Saddam possessed banned weapons.

Bush spoke out in Blair's defense on Wednesday, telling reporters the prime minister -- Bush's closest ally in the Iraq war -- "operated on very sound intelligence, and those accusations are simply not true."

On Tuesday, the House of Commons Foreign Affairs Committee began an inquiry -- broadcast live -- into the intelligence assessments, and heard from two former senior Cabinet ministers who accused Blair's government of exaggerating the danger represented by Saddam's alleged weapons of mass destruction.

The House Intelligence and Security Committee plans its own, closed-door probe as well.

Speaking to lawmakers, Blair rejected a call for an independent inquiry rather than the intelligence committee probe.

"I hope that when the truth is finally told by that committee, we will then have a debate on the basis of evidence and not on the basis of speculation -- the vast bulk of which is completely untrue," he said.

Home Secretary David Blunkett told the British Broadcasting Corp. on Wednesday that the two ministers who testified a day earlier -- Robin Cook and Clare Short -- hadn't had access to all the intelligence on Iraqi programs. Blair's main argument for going to war in Iraq was based on those claims.

The committee inquiries will look in particular at two dossiers about Iraqi weapons published by the government before the war.

The first, published in February, has since been found to be substantially copied from an American researcher's thesis available on the Internet. The other in September made the claim -- since disputed -- that Iraq could fire chemical or biological weapons within 45 minutes of Saddam giving an order to do so.