WASHINGTON - The campaign to eliminate Iraq's most deadly weapons officially begins today when 18 United Nations inspectors are scheduled to arrive in Baghdad toting thick dossiers on hundreds of potential weapons sites, from warehouses and clinics to breweries and petrochemical plants.

The team plans to make its first inspection Wednesday, when it will scour an undisclosed site for tell-tale equipment, chemicals and documents that could provide clues that Iraq has rekindled covert biological, chemical and nuclear programs since 1998, when U.N. inspectors last withdrew.

These initial searches will probably involve well-known sites long associated with Iraq's weapons programs, and are expected to be essentially warm-up exercises unlikely to produce confrontations or much evidence, according to U.N. officials and other arms-control experts.

But in the coming weeks, the inspections will become increasingly aggressive and less predictable as the team gains experience, expands its fleet of jeeps and German helicopters and grows to its full size: 80 to 100 people by the end of the year. The team is led by Hans Blix, an experienced veteran of inspections by the International Atomic Energy Agency, but includes many people with less experience, including some who have never been to Iraq.

"I think Blix is under immense, quiet pressure from the United States," said Anthony Cordesman, an Iraq expert at the Center for Strategic and International Studies in Washington.

"If he doesn't go to core inspection areas quickly, he understands he will be in a quiet confrontation with the United States," Cordesman said.

What concerns American and U.N. officials most are two potential Iraqi innovations for hiding weapons: mobile biological weapons labs and underground or urban facilities for chemical and nuclear weapons.

Weapons experts say the new urban sites are probably housed in plain warehouses and commercial buildings in densely populated areas, where they would be harder to detect by spy satellites and somewhat shielded from American bombs.

"It would be like something from The Man From UNCLE, where you go in a plain storefront and suddenly find yourself in a weapons lab," said David Albright, a former nuclear weapons inspector who is president of the Institute for Science and International Security in Washington.

The inspectors will be following a three-part strategy, former inspectors and U.N. officials say. First, they will search for clear evidence of weapons production that could lead directly to charges that Iraq is in "material breach" of U.N. Resolution 1441 requiring it to disarm.

Yesterday, as expected, the Iraqi government complained to the United Nations that the small print behind the weapons inspections will give Washington a pretext to attack, the Associated Press reported.

Given President Saddam Hussein's expertise at hiding weapons, officials say, it is more likely that violations will be documented incrementally, through painstaking detective work that could take months.

To that end, inspectors will be documenting two other types of evidence: patterns of deceit and attempts to obstruct inspections. These could range from disabling jeeps to destroying documents to refusing to account for weapons materials that inspectors are certain exist.