CHINA ONCE again has been helping another country with a highly suspect nuclear project. Over the years the Chinese have provided much assistance to Pakistan in its pursuit of nuclear weapons. Now they have been working in Algeria on a nuclear reactor.

This reactor is too large to be purely a research machine. Since there seems to be no generating equipment on the site, it's evidently not being built to generate electricity. That leaves the possibility that its purpose is to produce the ingredients of nuclear weapons.

Both China and Algeria have naturally denied that they have anything of the kind in mind. But beyond that, they offer no explanation. The world -- or most of it, at least -- depends on a system of inspections by the International Atomic Energy Agency to police the rules against illicit bomb-building. But the IAEA has not been allowed to have a look at the Chinese work in Algeria. China is a member of the IAEA, and in 1984 it promised that it would export nuclear technology only under international inspection. Its Algerian activities seem to bring that promise into deep doubt.

The United States has only one sanction to use against China, but it is not a trivial one. Currently this country gives China what's known as most-favored-nation status for its exports here. It means that Chinese goods enter the United States under tariffs as low as the lowest enjoyed by any other country. Chinese sales to the United States have been increasing rapidly. But the president has to renew this status annually, by the end of June. If he should decline to do that, or if Congress should vote to overrule him, China's trade here would run into very substantial tariff barriers.

Recently there have been a couple of indications that Mr. Bush's previously impervious sympathy for China may be fading. Several weeks ago the White House began to express its growing irritation over China's trading methods and its interference with American attempts to export there. Last week, over vehement Chinese protests, Mr. Bush invited the Dalai Lama in for conversation about human rights violations in Tibet.

Now there's the revelation of China's part in building the Algerian reactor, apparently a clear breach of explicit assurances that China gave this country only a few years ago. If China had perversely set out to make it impossible for Mr. Bush or Congress to extend its most-favored-nation status, you'd have to say that it was doing all the right things.