IN a week of extraordinary developments on the nuclear-weapons front, none will have more lasting significance than yesterday's 178-nation decision to give the 1968 Nuclear Non-Proliferation Treaty (NPT) a more permanent life.
''It's a great triumph,'' says Spurgeon Keeny, president of the Arms Control Association in Washington. ''The decision for indefinite extension establishes nonproliferation as a permanent norm and not a temporary arrangement subject to future political developments.''
The treaty -- in which nonnuclear states agreed not to acquire nuclear weapons and declared nuclear states agreed to try to phase out their own -- is widely regarded as a cold-war success story. Only India, Israel, and Pakistan have acquired and maintained nuclear weapons since 1970.
But even as a victory for nonproliferation was being won at the end of a three-week conference at the United Nations in New York, there were mixed achievements elsewhere.
In Moscow, President Clinton and Russian President Boris Yeltsin agreed this week to push for immediate ratification of the START II treaty, which would slash the strategic forces of both countries. But Mr. Yeltsin declined to call off a deal with Iran to complete two nuclear reactors that, US officials say, could speed the day the pariah state acquires its own nuclear arsenal.
Elsewhere, a deal between the United States and North Korea to curb that country's nuclear-weapons program hangs in the balance.
In Washington, meanwhile, a report issued this week by the Carnegie Endowment for International Peace confirms that even as China has lobbied to extend the NPT, Chinese nuclear officials have been helping Pakistan complete construction of an unsafeguarded facility that would be the source of plutonium for nuclear weapons.
''Despite the good news in New York, we're far from being home free,'' says Mr. Keeny.
The decision to extend the NPT comes after months of lobbying by the US and other declared nuclear states -- Britain, France, Russia, and China -- which have argued that the treaty represents the best hope to avoid nuclear war.
The final agreement was delayed after Arab states, led by Egypt, drafted a resolution singling out Israel for its failure to join the NPT. Israel is the only state in the Middle East that possesses nuclear weapons. Compromise language agreed to late Wednesday calls on all nations that have uninspected nuclear programs to abide by the treaty.
US officials were pleased by the breadth of support that was finally garnered for indefinitely extending the treaty, which was approved yesterday after the president of the review conference, Sri Lankan Ambassador Jayantha Dhanapala, announced that a consensus had been reached.
US officials were also surprised by the broad support indefinite extension received from small, nonaligned nations, which have no prospect of ever acquiring nuclear weapons and whose main concern is the real or potential nuclear threat posed by large regional powers like Iran or India.
In the run-up to the conference, some states favored a series of fixed extensions for the NPT, as a way of maintaining leverage on the nuclear states to disarm.
In what amounts to a quid pro quo for permanent extension, the nuclear powers were expected to back agreements that would call for, among other things, more frequent treaty reviews, a comprehensive ban on nuclear testing by the end of 1996, and the eventual elimination of nuclear weapons, which is also called for in the treaty itself.
Whatever their commitment to the principle of eventual elimination, US officials are in no mood to commit to a specific timetable until major treaties like START I and START II are fully implemented and as long as states like Iran have potential or actual nuclear weapons capabilities.
''Nuclear weapons are absolutely essential for our position in the world and our security,'' says one arms-control expert.
Despite the pledge by Clinton and Yeltsin to press for early ratification of START II, there are growing doubts in the US that the Duma will go along. Nationalists in the Russian parliament say the treaty favors the US by cutting out the heart of Russia's missile defense -- its heavy, land-based, multiple-warheaded strategic missiles.
Reluctant in Moscow
The failure to persuade Russia to abandon its nuclear deal with Iran has been especially disconcerting to Clinton officials, who have shared intelligence information with Moscow that, they say, proves that Iran is bent on developing nuclear weapons.
''You have to assume that they don't accept the information or don't take it seriously,'' says John Holum, head of the US Arms Control and Disarmament Agency (ACDA). ''I think the latter is more likely.''
The deal has also inflamed critics in Congress, one of whom -- Texas Senator and Republican presidential aspirant Phil Gramm -- warned this week of the danger of having an ''atomic Ayatollah'' in charge of Iran.
But according to Leonard Spector, a nuclear-nonproliferation specialist at the Carnegie Endowment, ''Part of the reason that India, Israel, and Pakistan are not declaring that they have bombs, are not conducting nuclear tests, and are not brandishing their weapons has to do with this treaty.''
Another bulwark against proliferation is a 1976 agreement among nuclear states not to export weapons-related material to nonnuclear states. In Moscow this week, Yeltsin pledged not to sell a uranium enrichment gas centrifuge to Iran that would have violated the agreement.
The decision to extend the NPT was taken just days after the last installment of SS-18 nuclear warheads was returned to Russia from Kazakhstan. Kazakhstan is the first of three former Soviet states expected to completely denuclearize.
''It is one of the most important national security achievements since the end of the cold war,'' says Sen. Richard Lugar (R) of Indiana of the little-noted event.
In a related development, the United Nations Association issued a study yesterday calling on the UN Security Council to take a more assertive role in preventing the spread of nuclear, chemical, and biological weapons to rogue states.
The report calls for the appointment of a Special Rapporteur to provide the Security Council with a mechanism for early warning and preventive diplomacy.
Amid an intense week of proliferation-related activity, the House International Relations Committee in Washington voted along party lines Wednesday to abolish ACDA, which has played a major role in the NPT review conference.
Under the House measure, the functions of the agency, and those of the US Information Agency (USIA) and the US Agency for International Development (AID) would be folded into the State Department. A similar proposal is being weighed by the Senate Foreign Relations Committee.