During the next month, the United States is expected to come under intense pressure to match the Soviet Union's offer for a temporary ban on nuclear testing, as representatives from 100 nations meet in Geneva to assess progress in halting the spread of nuclear weapons.

The Soviet Union declared a halt to all underground nuclear tests for five months from Aug. 6, the 40th anniversary of the bombing of Hiroshima. The United States spurned the offer to join in a mutual test ban as a propaganda ploy, but many nations see the proposal as a rare opportunity to slow the deployment of nuclear weapons and spur arms control.

The five-year review of the 1968 Treaty on Nonproliferation of Nuclear Weapons, which opens Tuesday, has evolved into an important arena, not only for the enduring duel between Moscow and Washington for global political sympathies, but also for an increasingly emotional battle between the nuclear haves and have-nots.

The treaty, monitored by the International Atomic Energy Agency in Vienna, is considered the most critical diplomatic means of preventing a profusion of nuclear arsenals around the world, with 125 countries agreeing to abide by safeguards on the use and transfer of fissionable materials.

In the courting of public opinion prior to the November summit between President Reagan and Soviet leader Mikhail Gorbachev, the treaty-review conference will provide a highly visible forum for international sentiment favoring a range of arms control initiatives opposed by the Reagan administration.

The conference is expected to resound with new calls for a nuclear weapons freeze, a renunciation of first-use strategy, a disavowal of an arms race in space as well as a moratorium on nuclear tests. The Soviet Union has embraced all of those proposals, while the United States has rejected them.

Yet both superpowers, along with other nuclear-armed states, will be criticized strongly by neutral and nonaligned nations for failing to adhere to what was deemed the essential bargain of the treaty: if nonnuclear states vowed to block the "horizontal" spread of nuclear weapons to other countries, the nuclear powers were to stop "vertical" proliferation.

The treaty's Article Six obligated the nuclear powers "to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament."

While Third World countries have stressed this view at the previous review sessions in 1975 and 1980, the absence of progress in arms control during the past five years has exacerbated resentments. The neutrals argue that while no new nuclear-armed state has emerged during the past 15 years, the superpowers have expanded their nuclear arsenals steadily.

Some officials warn that the depth of frustration felt by many nonaligned states at the lack of progress toward arms control should not be underestimated. "Behind the rhetorical speeches you will hear is real bitterness over the fact that these countries feel cheated because the nuclear powers have not moved any closer toward curbing their nuclear arsenals," said a senior IAEA representative.

Some delegates have warned that if a confrontation between the nuclear and nonnuclear states comes to a head, a few countries might be prepared to withdraw from the treaty to dramatize their protest that Moscow and Washington have not lived up to their promise of working toward disarmament. Such a development could snowball, weakening adherence to the treaty and chances for its renewal once it expires in 1995.

Many nonaligned states also are dismayed that the nuclear powers have not fulfilled pledges to share their knowledge of exploiting nuclear energy for peaceful purposes. Tight restrictions on the flow of nuclear technology is one of the few areas of close cooperation remaining between Moscow and Washington.

Among the five known nuclear powers, Britain also has joined the United States and the Soviet Union in advocating strict controls on nuclear systems. France and China have not signed the treaty, ostensibly because it discriminates against states without nuclear weapons.

American and Soviet representatives have met five times in the past two years to coordinate their positions on proliferation questions.

"It's expected that there will be a lot of tough language used and a lot of tough questions asked," a senior U.S. disarmament official said. "We hope the strong mutual interest we have with the Soviets will carry the day."

There is a consensus among member states that the IAEA's safeguards system, carried out on a budget of $30 million, has proved highly successful in limiting the spread of nuclear weapons. About 98 percent of all nuclear installations in nations without atomic weapons are under the safeguards.

"At the beginning of the 1960s, president Kennedy prophesied a world of between 15 and 20 nuclear weapons states," Hans Blix, the IAEA's director general, said. "Yet the number has not increased since 1964, and today there is not a single new state openly professing a desire to develop nuclear weapons capacity."

The effectiveness of the treaty and the safeguards system is shown, IAEA officials say, in the fact that the greatest proliferation risks are posed by those countries that refuse to sign the treaty. India, Pakistan, Israel and South Africa are considered the most troublesome countries because they will not accept safeguards on key installations such as enrichment plants.

All four are labeled "phantom proliferators" because they are well advanced on the road to making nuclear explosives. Brazil and Argentina are judged to be further away from making the bomb. Moreover, IAEA officials believe that the Latin American states have shown signs of an increasing willingness to submit to inspections as a way of restraining each other's nuclear weapons potential.

The central basis of the safeguards lies in on-site inspection, something the Soviet Union consented to this year for the first time. Earlier this month, IAEA inspectors were allowed to visit the Novo Voronezh plant as well as a research reactor near Moscow.

Western diplomats and IAEA officials believe the Soviet decision to allow inspectors into its civilian nuclear reactors is a signal that the Kremlin may be prepared to accept on-site inspection of military installations. Moscow's past refusal has stalled arms control negotiations in which the United States demanded effective means of verifying compliance with any accord.