WASHINGTON (AP)—The United States has intelligence information suggesting that China is planning to set off a nuclear test explosion, an administration official said Thursday.

Such a test, which would be the first conducted by any country in a year, would threaten the 15-month moratorium declared July 3 by President Clinton on U.S. nuclear testing.

In announcing the suspension, Clinton warned that if any country tested its weapons he would seek authority from Congress to resume U.S. tests.

Russia, France and Britain have all promised that they would not be the first to test. But China has been sending what U.S. officials describe as contradictory signals.

On the one hand, China has agreed to take part in negotiations starting next January in Geneva to ban all nuclear tests by the end of 1996. On the other hand, China has refused to say categorically that it will not test in the meantime.

A high-level U.S. delegation that visited Beijing in July failed to elicit such a promise, as have subsequent contacts with the Chinese.

Now, said the U.S. official who spoke only on condition of anonymity, satellite information and seismic monitors suggest that China is getting ready to test at its Lop Nor desert site in northwestern Xinjiang province.

China conducted two tests last year—one in May, the other in September.

Western experts estimated that the May explosion had a one-megaton yield—equal to about 70 bombs of the strength dropped on Hiroshima.

The United States and former Soviet Union have an agreement not to test any bomb with a yield of more than 150 megatons. China hasn’t joined that pact.

China is believed to have the smallest arsenal of the world’s five declared nuclear powers—slightly less than Britain’s.

According to the Natural Resources Defense Council, a private research group that closely tracks nuclear weapons proliferation, China has over the years detonated 37 tests—compared with 936 for the United States.

A Chinese test ‘‘would have pervasive ramnifications,’‘ said Rep. Mike Kopetski, D-Ore., who led the drive in the House to win a U.S. testing moratorium. ‘‘If this is true,’‘ he said, ‘‘it would not only dramatically affect our moratorium’‘ but could prompt France to resume testing.

France’s Socialist President Francois Mitterrand promised last year to stop testing—to the chagrin of some in the French military and defense establishment. But U.S. officials became concerned that he might not be able to keep that pledge since he was forced to take a center-right partner into his coalition.

If France and the United States start testing, Kopetski said, ‘‘it would open up this whole madness again.’‘

Russian President Boris Yeltsin is not inclined to resume testing, and Britain is bound by any use decision because it can only test its weapons at the U.S. site in Nevada.

In London, the private Verification Technology Information Center said Thursday that it has evidence that a Chinese test may be imminent. The group, which successfully predicted China’s two 1992 tests, uses commercially available satellite photos on which to base its information.