The failure of US and British forces to find any weapons since they invaded Iraq 11 weeks ago has forced US President George W. Bush and British Prime Minister Tony Blair to deny reports that they fabricated an excuse for going to war.

Blix told the United Nations Security Council that "there remain long lists of items unaccounted for" in the nuclear, chemical and biological weapons programmes which Iraq claimed to have dismantled more than a decade ago.

"But it is not justified to jump to the conclusion that something exists just because it is unaccounted for," he added.

It was the kind of phrase that caused the Bush administration to express increased frustration with Blix in the weeks leading up to the invasion of Iraq on March 20, and to bar his inspectors from returning to Iraq since then.

"The lack of finds could be because the items were unilaterally destroyed by the Iraqi authorities, or else because they were effectively concealed by them," Blix said.

But he noted that there was a "new environment in Iraq, in which there is full access and cooperation, and in which knowledgeable witnesses should no longer be inhibited to reveal what they know," and he added: "It should be possible to establish the truth we all want to know."

Blix told the council the briefing was "likely to be my last." The 75-year-old former Swedish foreign minister retires as chairman of the UN Monitoring, Verification and Inspection Commission (UNMOVIC) at the end of this month.

Set up in December 1999, UNMOVIC was about to carry out only 15 weeks of inspections in Iraq before it was withdrawn on March 17.

Blix began by stressing that UNMOVIC "has not at any time during the inspections in Iraq found evidence of the continuation or resumption of programmes of weapons of mass destruction or significant quantities of proscribed items."

The one exception was the Al-Samoud 2 missile system, which Iraq declared to the inspectors and Blix concluded was banned because in test flights it had exceeded the 150-kilometre (93-mile) limit set by the council.

Blix said UNMOVIC had supervised the destruction of 50 of the 75 missiles which had been declared as deployed. But he noted that "50 percent of the declared warheads and 98 percent of the missile engines remained intact" when the inspectors left Iraq.

Security Council Resolution 1483, which lifted UN sanctions against Iraq on May 22, left the inspectorate in a kind of legal limbo.

The resolution cut off its only source of funding -- a 0.8 percent slice of Iraq's oil revenues -- but left intact some 200 million dollars in accumulated revenue, held in a UN-administered escrow account.

The resolution said the council would "revisit the mandate" of UNMOVIC as well as the inspection operations of the International Atomic Energy Agency.

Arguing for a future role for UNMOVIC, Blix told the council that "an effective presence of international inspectors will serve as a deterrent against efforts aimed at reactivating or developing new programmes of weapons of mass destruction" in Iraq.

After hearing Blix's report, the council moved into consultations with him behind closed doors.

WAR.WIRE