U.N. weapons inspectors gearing up for a return mission to Iraq will have to overcome daunting obstacles to shed light on Saddam Hussein's nuclear program, the chief nuclear arm inspector said Thursday.

Four years after they were pulled out of Baghdad, the International Atomic Energy Agency's core team of 18 nuclear inspectors will rely heavily on new sleuthing technology if they're deployed to uncover evidence Saddam may have concealed, head inspector Jacques Baute told reporters.

Although the nuclear inspectors have been in and out of Iraq since 1991, enduring sandstorms, scorpions and subterfuge, the stakes have never been higher: Their findings could stoke -- or undermine -- the U.S. effort to galvanize global support for an invasion.

"We're like policemen trying to find one murderer among millions of people," Baute conceded. "The probability seems quite low. The group of inspectors is small, while the country is quite big."

"But if you use the right techniques, the chances become quite good," he added. "A nuclear program needs a large infrastructure. That's something that benefits us."

The Vienna-based IAEA, the United Nations' nuclear watchdog, put its team on alert after Iraq's surprise announcement this week that it would accept the inspectors' return.

The agency says the team could leave as soon as the U.N. Security Council clears the mission and visa and travel arrangements are nailed down.

In New York, Hans Blix, the chief U.N. weapons inspector, told the Security Council Thursday that if all goes well at talks scheduled with the Iraqis in Vienna for Sept. 30, he could have an advance team on the ground by Oct. 15 and that some early inspections could be conducted soon afterward.

With council approval, Blix later told reporters, "will go there with a small advance team as soon as possible. We will select some sites that we think are interesting to go to in the early phases, so it's not like it takes two months before we can send any guys out there in the field. It will be much earlier than that," Blix said.

Blix had recently said his inspectors would need several weeks from the time they arrived in Baghdad until monitoring could actually begin.

But according to diplomats who spoke on condition of anonymity, Security Council members told Blix in a closed-door meeting Thursday that they would support a speedier timeframe for inspections to get under way.

The inspectors come from a dozen countries -- the United States, France, Britain, Russia, China, Ireland, Egypt, Austria, Canada, India, the Netherlands and the Philippines -- and will draw support and intelligence from other U.N. member states.

They include veteran physicists like Baute, who has spent years assessing Iraq's clandestine nuclear program. A separate New York-based team will head the hunt for biological and chemical agents.

U.N. weapons inspectors arrived in Iraq following the 1991 Gulf War under Security Council resolutions that tied Iraq's disarmament to the lifting of punishing U.N. sanctions slapped on Iraq after it invaded Kuwait in 1990.

The inspectors left Baghdad in December 1998 amid Iraqi allegations that some inspectors were spying for the United States and countercharges that Iraq wasn't cooperating with the teams. Their departure was followed by four days of punishing U.S. and British airstrikes on Iraq.

This time, the team will insist that Iraq keep its promise of unfettered access to suspect sites and cooperate fully with an inspection regime that Baute said would be "very intense."

"It is the only way that we can pursue inspections to ensure that we are able to check every corner, every building, see any person, any document," said IAEA spokeswoman Melissa Fleming. "In that way we can be able to tell the world whether or not Iraq has indeed revived its nuclear program."

Saddam denied his country has any nuclear, biological or chemical weapons in a speech read Thursday by his foreign minister at the United Nations, and said inspectors would have unfettered access.

But at U.N. headquarters, the United States and Britain apparently told Blix that the speech made them even less confident that inspectors would gain the kind of access necessary to disarm the country.

Blix, who held preliminary talks with Iraqi officials in New York two days ago, said they didn't discuss the matter of so-called "presidential sites," the eight areas -- totaling 32-square-kilometers -- which are protected from surprise inspections under a 1998 deal between U.N. Secretary-General Kofi Annan and the Iraqi government.

Baute said the inspectors would be relying on new technology, powerful software and a support staff of hundreds of highly skilled analysts in their search for banned materials.

"We're not expecting to find a full-blown (uranium) enrichment facility," he said. "Small-scale operations are difficult to find. But there are plenty of nuclear operations that can induce leaks and leave traces in the atmosphere, soil and plants. We can find them."

The team will use two new gadgets it didn't have before: "The Ranger," a portable scope that can detect gamma radiation, and "Alex," a compact machine that can tell if a metal object is a potential nuclear component.

Inspectors also will be armed with global positioning systems and conventional radiation detection equipment. They will transmit data, still images and video back to Vienna for analysis via a secure digital link, Baute said.

If they deploy, the inspectors will operate under the assumption that nothing is off-limits, he said. "We have the legal right to go anywhere," he said.

Recent analysts' reports have said Saddam is eager to develop nuclear weapons, but cannot develop nuclear material on his own. If Iraq received the material from abroad, however, it could produce a nuclear weapon in months, an expert report this month said.

"Even if we do return there, it could take up to one year to draw definitive conclusions," IAEA Director-General Mohamed El Baradei told reporters earlier this week. Even preliminary findings will take several months, Baute said.

By the end of the Gulf War, IAEA assessments indicated Saddam was six months away from building an atomic bomb. Inspectors discovered that the oil-rich nation had imported thousands of pounds of uranium, some of which was already refined for weapons use, and had considered two types of nuclear delivery systems.

Over the next six years, inspectors took custody of the uranium, destroyed facilities and chemicals, dismantled over 40 missiles and confiscated thousands of documents and plans.

"We try not to be aggressive, but we will be very persistent," Baute said.