The longer the United States and Britain occupy Iraq without finding weapons of mass destruction, the more conceivable it is that Baghdad destroyed them after the first Gulf War in 1991, chief United Nations weapons inspector Hans Blix said on Monday.

Blix, to retire next week after heading inspections before the US-led war on Iraq began in March, also spoke critically at a think tank meeting of one of Washington's key arguments for overthrowing Iraq President Saddam Hussein.

"It is sort of fascinating that you can have 100 per cent certainty about weapons of mass destruction and zero certainty of about where they are," Blix said at the Council on Foreign Relations in New York.

Two months after the fall of Baghdad in a war launched after the United States and Britain accusing Saddam of illegally harboring nuclear, chemical and biological weapons, their troops still have not found any such weapons.

"I'm simply saying that the longer we are in this situation without finding anything, the more we have to ask ourselves is it conceivable that they did destroy in '91," Blix told Reuters Television after the event.

Saddam said the weapons were destroyed in 1991 when a US-led international coalition ousted his army from Kuwait. Blix has said that inspectors made their last significant finds in 1994.

Blix's UN inspectors searched for weapons of mass destruction in Iraq from November 2002 to March 2003. Some US officials faulted the Swede for failing to produce tougher reports on Iraq's purported weapons.

But on Monday, Blix took aim at the Bush administration's assertions that Washington needed more time to find Iraq's weapons.

"Three-and-a-half months for new inspections was a rather short time before calling it a day and especially when we now see the US government is saying that, 'look, you have to have a little patience, you know these things take time.' All right," Blix told his audience of foreign policy analysts, business leaders, academics and journalists.

Blix, retiring on June 30 after heading the UN Monitoring, Verification and Inspection Commission since March 2000, said he would like to write a 'nuanced view' of the period.