To trick U.N. weapons inspectors, Iraqi authorities hauled away prohibited materials, bulldozed weapons sites and intimidated Iraqi weapons experts -- in one case ordering a dozen scientists confined to a guesthouse, Secretary of State Colin L. Powell told the U.N. Security Council today in illustrating what he called "a policy of evasion and deception that goes back 12 years."

Powell made a series of new allegations about Iraqi behavior, at times linking President Saddam Hussein to specific tactics intended to defy U.N. weapons inspectors. He said that in one case Hussein ordered a death certificate issued for a scientist who was then sent into hiding. In another, Hussein ordered a warning be sent to Iraqi scientists that cooperation with the inspectors would be punishable by death, Powell said.

One of Powell's most dramatic new charges was that the Iraqi military distributed rocket-launchers and warheads filled with biological agents in western Iraq, where they were hidden in palm groves. Citing "human sources," he said orders were given to move the weapons every one to four weeks to prevent discovery. The Iraqi government denies it has any biological or chemical weapons.

Outside analysts said the credibility of Powell's case requires faith in U.S. interpretations of satellite photographs and intercepted conversations between Iraqi officials, as well as significant trust in the unidentified informants cited frequently by the secreatary.

But these experts put special significance on the newly released satellite photographs said to show deceptive activity at alleged weapons sites, noting that previous inspection teams uncovered similar efforts to hide or remove evidence. The intercepted telephone conversations were suggestive, they said, but not decisive.

The evidence of Hussein's deception was the "strong suit" in Powell's presentation, said Gary Milhollin, director of the Wisconsin Project on Nuclear Arms Control. "He really presented an overwhelming case. There have been numerous examples in the past of Iraqis cleaning out inspection sites."

Powell displayed a satellite image of a truck caravan at a facility he said was related to germ warfare. He said the image, taken two days before U.N. inspections resumed, revealed activity at a regularly monitored site where such movement is rare.

A photograph taken the same day showed five cargo trucks and a crane at a missile facility, which Powell described as evidence that Iraq intended to move the missiles out of the range of inspectors. A third image, he said, showed a cargo truck preparing to move ballistic missile components. By Powell's account, another showed special vehicles and chemicals being taken away from a chemical weapons facility before inspectors arrived on Dec. 22.

Powell said U.S. analysts saw "this kind of housecleaning" at 30 sites, although he acknowledged that they did not know what Iraq was moving at all of those sites. He also reported that computer hard drives had been inexplicably replaced at Iraqi weapons facilities and said that Hussein's son Qusay ordered the removal of banned weapons from his father's many palaces.

The amount of deduction in some of Powell's examples, some analysts said, detracted from the overall strength of his argument. "I love imagery," said a former senior U.S. intelligence official, "but I don't know what I saw."

But David Kay, the former chief U.N. nuclear inspector in Iraq, said Powell had effectively woven together information from human sources and intercepts.

In one conversation, an Iraqi general tells a subordinate to make sure a "modified vehicle" from the Al-Kindi factory in Mosul is removed before inspectors arrive the next day.

The conversation rings true, Kay said, because inspectors learned of a very similar exchange that preceded a visit to a facility in September 1991. The Iraqis failed to removed critical evidence in that case, however, leaving inspectors with a bonanza of documents on Iraq's nuclear program.

"My initial reaction was Yogi Berra's 'deja vu all over again,' " Kay said.

Arguing that Iraq has engaged in "a deliberate campaign to prevent any meaningful inspection work," Powell said that "many sources" corroborated information that Hussein participated directly in the effort to prevent inspectors from interviewing Iraqi weapons scientists. He said Hussein decreed that any scientist who agreed to leave Iraq to meet inspectors would be treated as a spy.