An Iraqi scientist submitted to a private interview with weapons monitors for the first time Thursday, meeting a key U.N. demand hours after the chief nuclear inspector demanded a "drastic change" in Baghdad's level of cooperation.

The United Nations confirmed that a private, 3 1/2-hour interview had taken place with a biologist at the Al-Hayat hotel, where the inspectors reside, a senior U.N. official said.

But the concession by Baghdad was unlikely to satisfy Washington.

"The game is over," President Bush declared at the White House later Thursday. "Saddam Hussein will be stopped."

Bush accused Saddam of recently authorizing commanders to use chemical weapons in the event of war. He demanded quick action to disarm Iraq. Bush said he would be open to a second U.N. resolution on disarmament, following up one approved last November, but only if it led to prompt action.

Mohammed al-Douri, Iraq's ambassador at the United Nations, said in response: "It sounds like he wants a resolution for war."

Presidential adviser Lt. Gen. Amer al-Saadi first announced the scientist's interview at the end of a news conference where he made a point-by-point rebuttal of Secretary of State Colin Powell's presentation to the U.N. Security Council Wednesday indicting Iraq as a secret weapons builder.

"During the interview, a number of issues were addressed," U.N. deputy spokeswoman Hua Jiang said without elaborating.

Such private interviews have been among the biggest obstacles to progress in the U.N. inspection program.

A senior U.N. official said the interview indicated that the Iraqis were finally giving in to pressure, but "we don't know if this will be one attempt or lots will follow."

The official said he thought this scientist had been "properly tutored," or briefed, by Iraqi officials. "We accepted because we wanted to set a precedent," the official said.

Iraq has insisted it encouraged its experts to cooperate and that the scientists themselves refused to submit.

Hours before the interview was announced, one of the top inspectors, Mohamed ElBaradei, said in London that when he and his counterpart Hans Blix meet in Baghdad this weekend, Iraq needs "to show drastic change in terms of cooperation."

Blix told Fox television his "major impression" was that the Iraqis were trying to conceal things.

If they want to convince the inspectors they have no banned weapons, he said, they would be completely open and "would not make us feel that they are observing us and trying to lead us by the nose."

In Baghdad, Blix and ElBaradei will try to resolve practical issues, such as starting U-2 reconnaissance flights to aid inspections. They also want concrete Iraqi evidence to help close gaps in the accounting for old weapons.

In a lengthy meeting with journalists, al-Saadi analyzed in detail Powell's slide-and-audio presentation to the Security Council, and denounced its sources.

He said Powell was "quoting 'our sources,' 'our sources,' 'our sources,' without any convincing evidence, as if that in itself is enough to convince the world."

Al-Saadi, a chemist who once headed Iraq's advanced weapons programs, took each of Powell's assertions and presented a contrary explanation to what the secretary of state told assembled foreign ministers in New York.

Powell, for example, displayed satellite photos of alleged chemical weapons bunkers at al-Taji, just north of Baghdad, and said the photos showed that the area had been "sanitized" immediately before U.N. inspectors visited.

Al-Saadi said the bunkers were not suitable for chemical weapons, and that suggesting they could be so easily sanitized if they held chemical weapons "is ludicrous."

He cited Powell's assertion, "from sources," that Iraq had dispersed biological weapons to its western desert, and that these warheads were being sheltered in palm groves and moved every one to four weeks. Al-Saadi rejected this, and said he remembered that the "palm groves" and periodic transfers had come from an account given by an Iraqi officer of how he had hidden and moved conventional missiles during the Gulf War.

"The fiction goes on. It goes on and on," al-Saadi said.

He detailed other examples of what he said were cases of fact being mixed with fiction, of elements taken out of context, of baseless interpretations of reconnaissance photos. He also dismissed audiotapes that Powell described as intercepts of monitored Iraqi conversations about weapons.

"One can concoct anything and fabricate anything in this regard, and they are no evidence at all," he said.

At the same news briefing, Saeed Mousawi, a senior official at the Iraqi Foreign Ministry, said the Al Qaeda-Iraq connections Powell alleged on Wednesday are "false and poor allegations" presented to justify an American attack.

He said Jordan informed Iraq that Abu Musaab al-Zarqawi -- whom the United States describes as a suspected assassin and an Al Qaeda affiliate living in northern Iraq -- entered Iraq with a number of his associates using fake names.

"We also assert that a meeting of any Iraqi side -- either official or security -- with [al-Zarqawi] never took place," Mousawi said.

The Associated Press tried to contact Jordanian officials to confirm Mousawi's statements but none could be reached because Thursday is the beginning of the weekend there.

The next pivotal point in the long-running Iraq crisis will come when the chief inspectors file update reports Feb. 14 to the Security Council on Baghdad's cooperation in their efforts to verify that this country is free of chemical, biological and nuclear weapons programs, as mandated by U.N. resolutions.