The first contingent of U.N. inspectors landed here this evening pledging to push hard in scouring Iraq to determine whether President Saddam Hussein's government still possesses weapons of mass destruction or has revived secret programs to develop them.

The inspectors said they plan immediately to assess Iraq's pledge to give international experts unfettered access to any site they wish to visit, a daunting assignment whose results could decide whether the Bush administration launches a war against Hussein's government.

Meanwhile, the chief U.N. inspector, Hans Blix, told the Security Council at U.N. headquarters in New York that he has informed Iraq he will exercise his right to inspect Hussein's presidential compounds and other sensitive sites. But he also noted that Iraq, while pledging cooperation, insisted that inspections of the presidential sites and government ministries could not be conducted in the same manner as those of other facilities. [Details, Page A25.]

"We've had a lot of promises of cooperation," said Melissa Fleming, a spokeswoman here for the International Atomic Energy Agency, or IAEA, which is coordinating the inspections with a special U.N. commission assigned to examine biological, chemical and missile programs in Iraq. "We believe that's a good start. But we have suspicious minds. We're here to test cooperation, among other things."

In an apparent reference to U.S. criticism of the inspectors' plan to begin with low-key searches, Fleming said the IAEA and Blix's U.N. Monitoring, Verification and Inspection Commission have "appealed to countries to give us the patience that is needed to carry out comprehensive inspections." She added: "We have a huge mandate. It's going to take time."

Iraq has declared it no longer possesses any weapons or missiles banned by U.N. resolutions. The U.S. government, however, repeatedly has said Iraq has secretly held on to chemical weapons, may have biological agents and is seeking to develop nuclear arms.

One of Iraq's two vice presidents, Taha Yassin Ramadan, said today his nation expects a U.S. attack but will nevertheless cooperate with the inspectors "to prove to the whole world that America's evil plan" is to dominate the Middle East and serve Israel's interests, "not search for the so-called weapons of mass destruction."

U.N. officials said the 17 experts who arrived today plan to carry out their first inspection Wednesday. Although the inspectors refused to divulge the location of the inspection, U.N. officials have said the initial searches almost certainly will occur at well-known sites long linked to Iraq's weapons programs, where the experts will check on cameras and other surveillance equipment installed by earlier groups of inspectors. Those visits, U.N. officials said, likely will result in little new evidence or confrontation, but will provide practice for the newly arrived inspectors, many of whom have never worked in Iraq before.

The Bush administration has urged the United Nations to quickly order intrusive searches of Iraq's most sensitive sites, including Hussein's palaces, which were largely off-limits to the international inspectors who worked in Iraq from 1991 to 1998. Administration officials said such inspections would provide an immediate test of whether Hussein will honor his promise to comply with a U.N. Security Council resolution, passed unanimously Nov. 8, that calls for inspectors to be given access to any person or place without having to ask permission or provide advance warning.

President Bush has threatened to force Iraq to disarm -- shorthand for a U.S. military invasion to destroy Hussein's government -- if it does not cooperate with the inspectors.

Fleming suggested the initial inspections, even if they may not garner much new evidence, will provide a crucial evaluation of Iraq's willingness to comply. "It's not only what we find or don't find," she said. "How they cooperate is also important."

She also took a firmer tone than two senior U.N. officials who visited Baghdad last week, saying the inspectors were skeptical of Iraq's claim that it does not possess weapons of mass destruction. "We don't believe words," she said. "We believe actions. We believe what we can see with our own eyes."

Blix and the IAEA director, Mohamed ElBaradei, refrained during their visit here from publicly pressuring Iraqi officials to comply and instead said they would seek to avoid unnecessary confrontations with Hussein's government.

U.N. officials said the inspectors plan to start slowly not because they are loath to challenge Iraqi officials, but because they lack the manpower and equipment to visit large new sites, including presidential palaces. Although the inspectors brought high-tech sensors, computers and other monitoring gear on their plane from a regional headquarters in Cyprus, they lack equipment for secure telephone calls and data exchanges with U.N. officials in New York.

The inspectors also do not yet have helicopters. For now, they must rely on a fleet of white four-wheel-drive vehicles, whose movements can easily be tracked by the Iraqi government.

Fleming said 35 more inspectors plan to arrive on Dec. 8, the deadline for Hussein's government to provide a complete account of its chemical, biological and nuclear facilities. She said about 100 inspectors will be on the ground by Christmas.

U.S. officials have indicated they want the inspectors to be ready to conduct searches on short notice of any site after Dec. 8. If Iraq omits from its report sites the U.S. government suspects are used in the manufacture of banned weapons, it intends to call for immediate inspections of those locations.

Under the U.N. resolution, Iraq is required to list all weapons factories as well as installations with civil and military uses, including petrochemical plants, agricultural facilities and medical labs. Western experts say Iraq has thousands of such sites.

U.N. inspectors first arrived in Iraq shortly after the end of the 1991 Persian Gulf War. They were credited with destroying a large portion of Iraq's chemical weapons stockpile and monitoring equipment that could be used in the manufacture of nuclear and biological devices. But the inspectors found themselves enmeshed in frequent disputes with the Iraqi government, which restricted their ability to travel and visit certain sites. The inspectors withdrew in December 1998, declaring that Iraq's obstructions made it impossible to carry out their mandate. The United States and Britain subsequently launched four days of airstrikes against Iraq.

Hiro Ueki, spokesman for the U.N. Monitoring, Verification and Inspection Commission, leads the first team of 17 weapons inspectors off a bus as they arrive at their hotel in the Iraqi capital.An Iraqi soldier closes the gate of a parking lot in Baghdad containing vehicles to be used by U.N. arms inspectors.Members of the first team of inspectors wait on a bus after arriving at Baghdad's Saddam Airport. About 100 inspectors will be in Iraq by Christmas.