U.N. arms experts entered a vast riverfront palace compound containing President Saddam Hussein's main office to search two high-security buildings today, intensifying their hunt for evidence of banned weapons programs less than two weeks before they must report their progress to the U.N. Security Council.

The nearly four-hour search of the grounds of the Republican Palace, a sprawling and secretive complex that serves as Iraq's White House, was the inspectors' most significant incursion into the nucleus of Hussein's government since they resumed their activities here seven weeks ago. Hussein uses the palace to receive foreign visitors and to hold formal government events.

Although the U.N. team stayed away from Hussein's residence and office, the buildings they searched suggested that the inspectors have begun acting upon intelligence data, some of which was supplied by the U.S. government, to identify new sites to visit. A U.N. official said the buildings, near the edge of the palace grounds, had never been searched by inspectors and had not been identified by the Iraqi government as related to current or past weapons programs.

Demetrius Perricos, a top official with the U.N. Monitoring, Verification and Inspection Commission who led the inspection, would not say how the buildings were chosen, but he noted that satellite imagery of the site showed the construction of double fences and high walls that were deemed suspicious. "You don't know what's under these roofs," he said. "It just shows that it's nicely protected."

In one of the buildings, he said, the inspectors discovered three large safes, which were opened after a two-hour wait for a person who knew the combination. The inspectors examined the documents found in the safes, he said, but did not remove anything, suggesting they found nothing incriminating.

After the inspectors left, a short, gray-haired man who called himself a civil servant and said his name was Abu Mohammed Issawi, said the U.N. team looked at a building that coordinates veterans affairs. A few moments later, a security official clad in a black leather jacket pulled Issawi away and told him to "stop it."

While one U.N. team was at the palace, 10 other groups fanned out across the country in one of the most active days of inspections since searches resumed on Nov. 27. Among the sites visited was a munitions depot in Tikrit, about 100 miles northwest of Baghdad and the home town of Hussein and many of his top advisers and most loyal troops.

Today's activities appeared to be, at least in part, a response to criticism from the Bush administration that the inspections have not been fast or aggressive enough. U.S. officials have urged the chief U.N. weapons inspector, Hans Blix, to conduct more intrusive searches and to take Iraqi scientists out of the country for questioning. On Tuesday, National Security Adviser Condoleezza Rice traveled to the U.N.'s New York headquarters to urge Blix to heed the U.S. requests.

Perricos, the U.N. official, said the inspectors had no difficulty entering the palace grounds today. They were kept waiting for 15 minutes, a delay he called "understandable," while Iraqi officials accompanying the inspectors sought approval from their superiors to open the gates.

The palace, ringed by high cement walls and plainclothes security personnel, is usually off-limits to everyone except senior government officials and Hussein's relatives, office staff and personal security detail. Motorists using a nearby expressway are prevented from stopping outside the gates. Although the complex is on the Tigris River in central Baghdad, some official maps depict the 1.7-square-mile compound as undeveloped swampland.

For most of the 1990s, Hussein's government refused to allow inspectors into the Republican Palace and other presidential sites on the grounds that it would violate Iraq's sovereignty. But in February 1998, after President Bill Clinton threatened to launch military strikes against Iraq, U.N. Secretary General Kofi Annan hammered out a compromise that allowed inspectors to visit the palaces and other sensitive sites if they provided advance notice and were accompanied by a team of diplomats. A team of inspectors and diplomats subsequently scoured the Republican Palace on two occasions in March and April 1998 but reported finding no evidence of weapons of mass destruction.

The Nov. 8 Security Council resolution that restarted the inspections calls for inspectors to visit any site they want, whenever they want and without having to ask for permission. On Dec. 3, the inspectors briefly searched one of Hussein's other palaces, the Sijood compound, a largely unused facility in Baghdad.

Iraqi officials protested the visit to Sijood, calling it "unjustified and unnecessary." Last week, Hussein accused the inspectors of gathering intelligence for foreign governments. And today, an influential newspaper owned by Hussein's son lashed out at the inspections in a front-page editorial, saying, "Iraqis are angry and agitated, and some of them can no longer tolerate the sight of inspectors' teams."

Iraqi officials stand near missile parts in a factory about 43 miles west of Baghdad as U.N. inspectors searched for banned weapons throughout Iraq.