The nuclear arms race between India and Pakistan has progressed with disquieting speed over the past year. Both countries continue to deny that they have nuclear weapons or intend to build them. It may well be true that neither actually has a bomb at this moment. But the evidence strongly suggests that both are working rapidly toward a capability to produce nuclear weapons very quickly -- and more than one or two of them -- in a crisis.

Last winter President Mohammed Zia ul-Haq of Pakistan announced that his country had succeeded in enriching uranium to the low level required for power reactors. That implies a capacity to enrich to the higher grade required for weapons. The work is being done at a facility that Pakistan has never opened to international inspection. In July ABC News reported that Pakistan had successfully tested the trigger device for a nuclear bomb. The following month the Indians said that they had started up their new research reactor -- an unusually large machine for research -- and the announcement emphasized its plutonium output. It is not open to international inspection. India demonstrated in 1974 that it knows how to produce an explosion, and its present supply of plutonium will be sufficient for 15 or more weapons a year.

The pattern of these events is laid out in detail by Leonard S. Spector of the Carnegie Endowment for International Peace in his book "The New Nuclear Nations," the second in a highly valuable series of annual assessments. Other countries will doubtless always have far fewer of these terrible weapons than the United States or the Soviet Union. But the chances that the others will use them, if they have them, are certainly no lower. Among the countries that either have built nuclear weapons covertly or have recently been workingward them, two -- Israel and South Africa -- are surrounded by hostile neighbors. Others are locked in sharp regional rivalries. Argentina and Brazil seem to have slowed down their respective military nuclear programs since they recently returned to elected government. But Indo-Pakistani tensions appear to be considerably less under control.

Mr. Spector points out that President Reagan -- contrary to the impression that his own words sometimes leave -- has been playing a useful role there. A year ago he sent Gen. Zia a letter apparently suggesting that Pakistan would jeopardize the flow of U.S. aid if it actually began producing weapons-grade uranium. When India's prime minister, Rajiv Gandhi, was here in June, Mr. Reagan persuaded him at least that the United States was seriously trying to restrain Pakistan.

And perhaps Pakistan won't build a bomb. But both countries are still working hard to put these weapons within their easy reach. By no means all of the world's nuclear dangers are under negotiation in Geneva.