The world´s top nuclear inspector said yesterday that it may take 12 months to discover whether Iraq has weapons of mass destruction - a view that is likely to irritate Washington hawks.

"It will take us probably around a year before we can come to a reasonable conclusion," Mohammed el-Baradei, head of the International Atomic Energy Authority, warned.

"We will be able to report progress as we go along but we are not keen to rush to a conclusion... We´d like to take our time and I hope the world will bear with us as we go through this difficult task," he said in a television interview.

Under the recent UN resolution, the inspectors are required to provide an "update" report to the security council by late January. US administration officials have indicated that if the inspectors´ work is very prolonged, they reserve the right to act before the UN sets out its conclusions.

Mr el-Baradei, an Egyptian who often takes a conciliatory approach, said the team in Iraq had found nothing untoward during its first four days of inspections at several sites.

Speaking on the BBC´s Breakfast with Frost programme, he said: "We are off to a good start but we are far from reaching a conclusion.

"I would like to avoid a war. War is not good for anybody but we have an important job to do and we have to do a thorough job. It really depends on absolute 100% Iraqi cooperation so the ball really very much is in the Iraqi court."

He said he had impressed on Iraqi officials the need to "come clean" with the inspectors.

"There is a light at the end of the tunnel for Iraq if it cooperates fully," he added.

"War could be avoided, sanctions could be suspended, but if they don´t come clean and we discover that there are omissions, there will be, as the security council says, grave consequences."

As he spoke, inspectors in Iraq paid a surprise visit to a disused airfield 20 miles outside Baghdad which had once ostensibly been used to launch crop-spraying aircraft.

More than a dozen helicopters, stripped of their motors, sat on the disused tarmac as the inspectors checked the site and journalists watched from beyond a distant fence.

The previous Unscom inspectors discovered in the 1990s that the airfield, at Khan Bani Sa´ad, had been used for testing the so-called Zubaidy device, designed to spray toxic bacteria from a helicopter or slow-moving plane.

After their four-and-a-half-hour visit yesterday, the experts left without comment.

Montadhar Radeef Mohammed, the Iraqi official in charge of the installation, told reporters later that the UN experts had checked seals and tags left by their predecessors.

They had also gone through all the offices and rooms on the site and made copies of computer files, but he said they found no prohibited material.

A document acquired by Unscom in the 1990s showed that Iraq had successfully field-tested Zubaidy devices in 1988, though Iraq claimed they were not effective.

Iraq eventually handed over some early versions of the device for destruction but the inspectors were unable to discover what happened to the 12 final-version devices that they believed Iraq had produced.

While complaining about the tough terms of the inspections, Iraq also maintains that the checks have so far vindicated its claim to be free from weapons of mass destruction.

A report issued by the official Iraqi News Agency, and carried by all Iraqi newspapers, claimed the UN experts had only succeeded in uncovering the "lies" of Tony Blair.

It quoted a foreign ministry official as saying: "The foot-and-mouth disease institute and al-Nasr company [inspected last week] were among the sites accused by the report of British prime minister Tony Blair in September 2002 of carrying out banned activities.

"But the results that the inspectors reached recently reveal the spuriousness of the allegations and lies propagated by Tony Blair and uncover his false accusations against Iraq."

In the southern no-fly zone, western warplanes killed four people in a strike on an Iraqi oil plant yesterday, according to local residents. (Guardian Unlimited © Guardian Newspapers Limited 2002 12/02/02)