The world's major powers, including America's closest allies, warned the United States today that failure to ratify the multinational nuclear test ban treaty would send a dangerous signal that could encourage other countries to spurn arms control commitments.

With the Senate scheduled to begin debating the treaty Friday, envoys from nearly 100 nations at a conference here, including Russia, China, Britain and Germany, expressed alarm that the United States appears to be on the brink of rejecting the Comprehensive Test Ban Treaty. The pact, which President Clinton signed in 1996, would prohibit nuclear test explosions worldwide.

Diplomats said British Prime Minister Tony Blair and French President Jacques Chirac will soon make rare personal appeals to the United States to approve the accord, prior to a possible Senate vote next week.

In Washington, it was unclear if a compromise would be reached to postpone a vote on the treaty. Both sides agree that the pact will be defeated if it comes to a vote on Tuesday or Wednesday as scheduled. In the latest blow to the accord's prospects, Sen. Richard G. Lugar (R-Ind.), an influential arms control advocate, declared his opposition.

Majority Leader Trent Lott (R-Miss.) was sticking to his position late today that a vote can be delayed only if the Clinton administration promises not to try to revive the treaty before the president leaves office. The White House has rejected that proposal, and Sen. Joseph R. Biden Jr. (D-Del.), the ranking minority member on the Foreign Relations Committee, said he is "not hopeful" that the vote could be postponed.

Here in Vienna, diplomats said that Blair and Chirac will urge American treaty opponents to forgo partisan politics and weigh the damaging impact a negative vote would have on U.S. leadership in the effort to halt the spread of weapons of mass destruction.

There was particular concern here that some non-nuclear countries would regard failure to ratify the treaty as a broken promise that would relieve them of the obligation to comply with key parts of another accord, the Nuclear Non-Proliferation treaty. That pact is considered the linchpin of international efforts to limit the spread of nuclear weapons.

International anxiety also has been compounded by new worries over U.S. efforts to escape constraints imposed by the Anti-Ballistic Missile (ABM) Treaty, which limits the ability of the United States to build systems to defend against missile attack.

Russia and China say it would destabilize the strategic balance if the United States built a missile defense system, because Washington could be tempted to attack others if it felt invulnerable to retaliation. That could trigger a new arms race as other nations sought ways to overwhelm missile defenses.

Many nations are surprised by the Senate's hesitation to approve the test ban treaty, in part because the accord is widely regarded abroad as locking in American nuclear superiority. Until recently, the treaty had gained strong momentum as the ratification process moved ahead and a world-wide sensor system was deployed to detect even the tiniest indication of a nuclear explosion.

More than half of the 44 nations with nuclear facilities whose ratification is necessary for the treaty to take effect have already done so. U.S. approval is deemed critical to persuade other nations, including Russia and China, to ratify. Even more important, India and Pakistan, who pledged to sign the test ban treaty under enormous international pressure, are said to be awaiting Senate action before making their final decision.

"It would be a highly dangerous step for the Senate to reject this treaty," said Peter Hain, Britain's minister of state for foreign affairs. "If the test ban treaty starts to unravel, all sorts of undesirable things could happen. It would send the worst possible signal to the rest of the world by giving a green light to many countries to walk away from promises not to develop nuclear arsenals." Hain and other delegates here spoke at a long-planned conference organized to discuss how to put the test ban treaty into effect.

German Foreign Minister Joschka Fischer said the rest of the world would be watching the Senate test ban vote closely because of its possible effect in eroding support for the non-proliferation treaty. "What is at stake is not just the pros and cons of the test ban treaty, but the future of multilateral arms control," Fischer said.

Diplomats fear that a failure to put the test ban treaty into effect soon would discourage some "threshold" countries -- those close to developing nuclear weapons -- from cooperating with intrusive inspections under the non-proliferation treaty. Such inspections are designed to prevent them from cheating and secretly developing nuclear weapons.

Jayantha Dhanapala, the U.N. undersecretary for disarmament affairs, said many countries agreed to a permanent inspection regime four years ago only on the basis of a written guarantee by the nuclear powers to negotiate and ratify a worldwide test ban as one of several key steps toward nuclear disarmament.

In a grand diplomatic bargain struck in 1995, the inspection program was made permanent for some 175 nations that have promised to forswear nuclear weapons. In exchange, the world's five declared nuclear powers -- the United States, France, Britain, Russia and China -- pledged to reduce nuclear arsenals and approve a treaty that would ban test explosions that help upgrade their weapons.

"If the Senate rejects ratification, it would send a very negative signal that will act as a brake on the momentum we have achieved to control the nuclear threat, because some countries would see this vote as a betrayal of a promise," Dhanapala said.

The head of the U.S. delegation, Ambassador John B. Ritch III, said a main theme of the Vienna conference has been international alarm over isolationist thinking that has spurred Senate opposition to the treaty. He said foreign delegates found it difficult to understand how the Senate could consider backtracking from a ban on nuclear explosions even though polls show as much as 80 percent of the American public support the treaty.

China's representative here said that U.S. failure to ratify the test ban treaty would be "a very negative development" and joined others in expressing concern that the United States is shunning its obligations on global arms control.

"I don't like to talk about any country exercising world leadership, but in this case we see that the United States must play a special role," Sha Zukang, China's top arms control official, said in an interview. Sha added that China is even more alarmed by U.S. efforts to develop a regional missile defense system than by the Senate's reluctance to approve the test ban treaty.

Boris Kvok, Russia's deputy chief of disarmament issues, said the U.S. decision on the test ban treaty would not affect the deliberations of Russia's parliament on the pact or alter his country's test moratorium. "But if the U.S. moves ahead with ballistic missile defense, it would be a disaster for strategic stability in Europe and the world. And we would have to start developing new weapons to correct this imbalance," Kvok said.