For the first time since the toppling of Saddam Hussein, Hans Blix, the chief UN weapons inspector, confronted the Americans openly yesterday, accusing the Bush administration of lacking credibility in its efforts to hunt down Iraq's banned weapons.

Mr Blix, 74, derided by Washington for his failure to find the "smoking gun" that would have convinced the UN to give legal backing to the war, also accused Washington and Britain of deliberately undermining his efforts before the war.

He warned the Security Council that only UN inspectors, and not the teams being assembled by America, would be able to provide an objective assessment of any materials that might be found in Iraq.

Mr Blix spoke out as the diplomatic blood-letting seen in the run-up to the conflict risked resurfacing with the first full discussion by the Council on the next steps in Iraq.

The Council's members sparred openly over the role of the UN in identifying weapons of mass destruction in Iraq. And Mr Blix, who could now be the biggest obstacle to the removal of sanctions, which George Bush is seeking, rubbed salt in the wounds. London and Washington had built the case for invading Iraq on "very, very shaky" evidence, he said. He referred to documents alleging that Iraq had imported uranium for nuclear weapons from Niger that he later revealed to have been faked.

"I think it's been one of the disturbing elements that so much of the intelligence on which the capitals built their case seemed to have been shaky," he said, hinting that Britain the US might have allowed the information to surface to undermine inspections.

Mr Blix would not rule out that evidence of banned weapons might yet be uncovered. But he added that it was "conspicuous that so far [US inspectors] have not stumbled upon anything evident". He cautioned the Americans to "examine everything critically", noting that some Iraqis might be motivated to claim more than they knew.

Even in Washington, officials spoke of fears that inspectors deployed by the US might never find evidence of weapons of mass destruction that constituted the main political justification for invading Iraq. US officials are worrying out loud that Iraqi agents might have been able to destroy incriminating materials in the days of chaos that followed the taking of Baghdad. Senior officials believe the US military might have contributed to the difficulties by failing to secure potential weapons or intelligence sites during the frenzied looting.

The new standoff in the Security Council is about whether UN inspectors, told to pack their bags and leave Iraq 24 hours before the first bombs fell on Baghdad, should be sent back in to identify any weapons finds now being made. The US, determined to keep the anti-war camp out of the decisions on Iraq's future, stands alone in resisting calls from other members, notably Russia, to send UN inspectors back.

The role of Mr Blix is directly linked to the issue of when UN sanctions on Iraq can be lifted. President Bush asked the UN last week to end the sanctions. But Russia has argued strongly that under UN resolutions, sanctions can only be lifted once Iraq is certified as weapon-free and that that can only be completed by Mr Blix.

Ari Fleischer, the White House spokesman, said yesterday: "We are looking forward, not backward. Saddam Hussein's regime is gone, and we will need to reassess the framework design to disarm the regime given the new facts on the ground." In a sideswipe at Mr Blix he said: "I think it's unfortunate if Hans Blix would in any way criticize the US at this juncture. The US is working with Iraqis to build a new country for them."

Peter King, a Republican congressman, flatly dismissed Mr Blix's claims, accusing him of "manipulating evidence".

John Negroponte, the US ambassador to the United Nations, said: "For the time being, and for the foreseeable future, we visualize that [inspections] as being a coalition activity," he said. "The coalition has assumed responsibility for disarming of Iraq."

This puts the US directly at odds with the remaining members of the Security Council. Even Britain is making behind-the-scenes efforts to argue the case for giving Mr Blix a role in looking for weapons and certifying that they have been eradicated or do not exist. France took other members by surprise by asking for an immediate suspension of UN sanctions on Iraq. That move may be designed to mend fences with Washington, which has also called for an end to the sanctions. But France is also insisting on the return of UN inspectors.

Mr Blix, who is said to be livid that the US is assembling its own inspection teams, said: "We may not be the only ones in the world who have credibility, but I do think we have credibility for being objective and independent."