UNITED NATIONS, AUG. 9 -- The Security Council today unanimously declared Iraq's annexation of Kuwait "null and void," raising the possibility of concerted international military action to enforce U.N. sanctions against Iraqi President Saddam Hussein's defiant government.

Speculation about such military action was fueled both by the strong language of the resolution, approved by all 15 council members, declaring that Iraq's announcement of annexation Wednesday "has no legal validity" and by a broad hint from the Soviet Union that it might join a military operation under U.N. auspices if Iraq remains intransigent.

The resolution said nothing about U.N. members taking military steps either collectively or individually, as the United States has done in dispatching forces to Saudi Arabia and other parts of the Persian Gulf area.

However, a senior U.S. official in Washington, speaking on condition that he not be identified, said the Bush administration clearly would prefer any military operations in the Persian Gulf to be "wrapped in the U.N. flag."

Today's resolution marked the third time in the past week that the council tried to press Iraq to end its invasion of Kuwait and respect the independence of its tiny neighbor. The move caused many diplomatic sources here to conclude that there is a consensus among most of the United Nations's 159 members that Iraq's action must be reversed even if it requires resorting to the threat of military force.

As evidence of this building momentum, the sources cited the signals from Moscow and the announcement by King Hussein of Jordan, normally a close ally of Iraq, that his country will cooperate with the economic sanctions that the council imposed on Iraq on Monday. Even in Iran, which long has opposed any U.S. role in the gulf, press reaction today signalled that some moderates in the government may accept "drastic action" by the United States against Iraq.

Arab League members gathering in Cairo put off the start of their summit until Friday, apparently attempting to negotiate a solution to the crisis in informal meetings. {Details on Page A25.}

The discussion here about the likelihood of collective military action appears to focus on two possibilities. One is a naval blockade to enforce the economic sanctions. The other would go further and create a multinational force, probably built around the U.S. troops in the region, to confront the Iraqi forces occupying Kuwait.

One question that has not been addressed publicly is the cost of such military action. However, Secretary of State James A. Baker III said in Ankara, Turkey, today that the Kuwaitis had "offered to do what they can" to cover financial losses, at least in the case of Turkey, which stands to lose hundreds of millions of dollars by complying with U.N.-imposed economic sanctions.

U.N. and other officials here stressed that any discussion of collective military deployment is highly speculative and said they knew of no actual plan being formulated for a U.N. military role in the gulf crisis.

When U.S. Ambassador Thomas R. Pickering was asked following today's vote about the possibility of a U.N. force, he replied, "Those are all decisions that President Bush will have to make, and the president has said nothing is ruled in and nothing is ruled out."

The official in Washington said the administration has been preoccupied with the logistics of getting U.S. forces to the region and is only now beginning to weigh the possibilities for U.N. military involvement. He added that 15 options are being weighed, that talks have started with the Soviets and that the United States may be ready to bring a proposal to the Security Council early next week.

He did not elaborate on what a U.S. plan might involve. Although the United Nations has been involved in many peace-keeping operations in its 45-year history, its only "police action" to deter agression occured in 1950 when it in effect delegated to the United States responsibility for fighting North Korea's invasion of South Korea.

U.N. officials noted that the Korea operation, coming at the height of the Cold War, was accomplished through a complex legal maneuver that circumvented a Soviet veto. To this day, the United Nations has never resorted to the provisions in its charter permitting the Security Council to authorize military action to deal with a threat to international peace.

However, the officials added, in the post-Cold War atmosphere now making itself felt in the world body, such a move may be possible now. The statement made in Moscow today by a Foreign Ministry spokesman stressed that the Soviet government insists that any action must be "through collective efforts and the utmost use of U.N. mechanisms."

While saying that the Soviet Union "opposes force and unilateral decisions," the statement added that Moscow is "prepared for immediate consultations within the framework of the U.N. Security Council's Military Staff Committee, which according to the U.N. charter can perform very important functions."

That was a reference to a little-known adjunct of the council whose titular members are the top-ranking military officers of the five permanent council members: the United States, the Soviet Union, Britain, France and China. In practice, those nations are represented on the committee by officers attached to their U.N. delegations. "For 45 years, their only function has been to gather once a month, shake hands and disperse," a U.N. official said today.

However, the Soviet statement's observation that the committee "can perform very important functions" was interpreted by many here as a reference to the charter's provision that designates the committee as the planning body for any military action against threats to world peace.

While U.S. and U.N. officials said they do not know what the Soviets have in mind, they said Moscow's call for consultations within the committee appears to show a willingness to discuss military action under U.N. auspices. The officials said it might be significant that the current chairman of the committee is a Soviet, Maj. Gen. Grigori Yakovlev.

Today's Security Council resolution was the first since the crisis began to win the backing of all 15 members. When earlier resolutions -- calling for immediate withdrawal of Iraqi forces from Kuwait and imposing economic sanctions -- were voted on, Cuba, an implacable foe of the United States, and Yemen, the only Arab country on the council, abstained.

Today, however, no member spoke in defense of Iraq. The council declared that "annexation of Kuwait by Iraq under any form and whatever pretext has no legal validity and is considered null and void."

Cuban Ambassador Ricardo Alarcon de Quesada said his government had "no difficulty" supporting the resolution. Yemen did not speak.

Setting a rhetorical tone echoed by several other speakers, Pickering reiterated the comparisons that senior U.S. officials have made between Iraq and the Nazi and Fascist aggression of the 1930s. He said:

"There is something repugnant, chilling and vaguely familiar about the statement issued yesterday by the Iraqi Revolutionary Council. We have heard that rhetoric before. It was used about the Rhineland, the Sudetenland, about the Polish Corridor, about Mussolini's invasion of Ethiopia and about the Marco Polo Bridge incident in China. . . . The world community did not react. The result was global conflagration. We here will not and cannot let this happen again."

Following the vote, Iraqi Ambassador Abul A. Anbari defended his country's action as necessary to undo the effects of the colonial era that split "the indivisible Arab nation into 22 states. . . . My government reaffirms that the unity between Iraq and Kuwait is an indestructible one."