UNITED NATIONS - President Bush demanded yesterday that the United Nations act boldly and quickly to disarm Iraq, warning that the international body's reputation was at stake and that yielding to Saddam Hussein's regime could render the United Nations "irrelevant."

Speaking at the General Assembly to officials of 190 countries, Bush said he would work with them - at least for now - to exert diplomatic pressure on Hussein. The Iraqi leader, the president said, must destroy his weapons of mass destruction and end his decade-long defiance of other U.N. demands.

Though Bush set no deadlines, he made clear that his clock is ticking. He implied that if the United Nations failed to act with haste - or if Hussein ignored any new U.N. resolution - the United States was ready to act alone to oust his regime, which Bush called a "grave and gathering danger."

"The purposes of the United States should not be doubted," he said. "The just demands of peace and security will be met - or action will be unavoidable. And a regime that has lost its legitimacy will also lose its power."

Many nations represented at the General Assembly, including most of America's allies, are wary of confronting Iraq militarily, at least before a strenuous diplomatic effort has been made. Bush's speech was an effort to show the world that the United States welcomed the idea of a multinational coalition to deal with Iraq.

White House officials said Secretary of State Colin L. Powell would begin working on a new resolution today with officials from Britain, France, China and Russia, which, along with the United States, are the permanent members of the Security Council. It would require Hussein to disarm or prove that he does not possess, and is not pursuing, chemical, biological or nuclear weapons.

One senior administration official said that while Bush "is not anxious to go to war," he is deeply skeptical that Hussein will ever abide by U.N. demands. He said Bush is prepared to use U.S. force, with or without backing from other countries.

Using stern language, Bush cast Hussein's decision to defy U.N. resolutions as a critical test of the United Nations' relevance. He argued that Iraq's pursuit of dangerous weapons is "exactly the kind of aggressive threat the United Nations was born to confront" and that now was a "defining moment" for the international body.

"Iraq has answered a decade of U.N. demands with a decade of defiance," Bush said. "Are Security Council resolutions to be honored and enforced or cast aside without consequence? Will the United Nations serve the purpose of its founding, or will it be irrelevant?"

Aides described Bush as insistent on a new Security Council resolution that would be tougher than the 16 resolutions the United States says Hussein has ignored over the past 11 years - a resolution that would require him to act quickly and allow no wiggle room to stall.

"This is not something of months," one top White House aide said. "This is something of weeks."

In a chamber full of diplomats and leaders, some of whom fear that a U.S. invasion of Iraq could destabilize the Middle East and splinter the international coalition that has formed in the past year to fight terrorism, Bush's 25-minute speech received restrained applause. A few delegations - including Iraq's - remained expressionless and did not applaud.

For now, Bush is pursuing a diplomatic course that many world leaders and lawmakers had urged. Still, he appears to be treating a Security Council resolution as a last-ditch diplomatic thrust, essentially a stop on the road to a likely confrontation to topple Hussein.

Some world leaders say they wonder whether a forced removal of Hussein would violate international law.

U.N. Secretary-General Kofi Annan, speaking before Bush yesterday, said that "individual states may defend themselves" if they are attacked, but that in all other cases, military force must be supported formally by a body such as the United Nations.

White House lawyers have argued otherwise, insisting that Bush would have the right to invade Iraq if he concluded that Hussein threatened Americans or U.S. interests.

U.N. nations, Annan said, should continue pressuring Iraq to comply with existing resolutions. Still, striking a relatively tough tone, the secretary general added that if Iraq's defiance continues, the country "must face its responsibilities."

Iraqi officials scoffed at Bush's speech and continued to deny that they possess weapons of mass destruction.

Bush "chooses to deceive the world and his own people by the longest series of fabrication that [has] ever been told by the leader of a nation," said Mohammed al-Douri, Iraq's ambassador to the United Nations. "We don't care about the position of the U.S. If they are threatening, if they would attack, certainly we will be there for defending ourselves."