The United States has detected traces of radioactivity in clouds outside the Soviet border on almost 200 different occasions in the last 23 years, assistant defense secretary Richard N. Perle testified yesterday.

In about 100 of those incidents, in which there were indications that radioactive gases and particulates had escaped from Soviet underground nuclear-weapons tests, U.S. officials secretly protested to Moscow, Perle told a Senate Foreign Relations subcommittee. The last protest, according to a State Department official, was in late 1984.

Another U.S. official said the "minute traces" of radioactivity were discovered because Soviet nuclear weapons test sites are located near that country's borders and U.S. monitoring equipment is highly sensitive. The official said the traces posed no health risk.

Between 1964 and 1970, 97 such weapons tests by the United States also resulted in release of radioactive gases, according to the Environmental Protection Agency. On 31 of those occasions, the clouds drifted outside the Nevada Test Site, a Department of Energy (DOE) spokesman said.

Since that time, three tests have released gases, most recently last month when a nuclear-effects test code-named Mighty Oak released minute amounts of radioactive xenon, according to a DOE spokesman.

Soviet officials have disputed the U.S. contention that release of radioactivity after Soviet tests violates any treaty, Perle said.

He also said the U.S. findings "were kept secret from countries" along the Soviet border and may have led to the Soviets being "rather cavalier" in the way they initially kept silent about radioactivity released in the recent Chernobyl nuclear power plant accident.

Perle's disclosure came amid a broad Reagan administration attack on congressional resolutions seeking ratification of the 1974 threshold test ban treaty and resumption of negotiations to end all nuclear tests.

Adm. William Crowe, chairman of the Joint Chiefs of Staff, told the subcommittee that the JCS no longer supports, as it has since 1977, ratification of the threshold treaty, which limits both superpowers to underground tests of 150 kilotons or less. That is the equivalent of 150,000 tons of TNT.

In response to a question from Sen. John F. Kerry (D-Mass.), Crowe said the chiefs do not support Senate ratification without additional verification measures being sought by the administration.

In August 1977, Vice Adm. Patrick J. Hannifin, director of the JCS staff at that time, told the Senate committee that the chiefs supported ratification. The only safeguard required, he said, was not associated with verification but one that would allow continued U.S. testing below the treaty limit.

Crowe said he believed that the Soviets are violating the treaty by testing above its limits. Under questioning, he said it is "possible they were getting advantages" by such actions, but "we don't fully understand" what they might be.

Crowe also spoke firmly against any agreement to ban underground nuclear testing. While other administration officials have emphasized the need for continued testing to maintain reliability, safety and security of the U.S. stockpile, Crowe emphasized the necessity to be able to develop new weapons, particularly if the two sides reach agreement on reducing current stockpiles.

Saying there are "major uncertainties surrounding the mix of [Soviet] launch vehicles apt to evolve from a nuclear disarmament process," Crowe underscored "the importance of remaining flexible in [U.S.] weapon system design and validation of such designs through testing."

In 1977, Hannifin said the chiefs believed that, while a comprehensive test ban might "inhibit some work," the military "with some less efficiency . . . could use current warhead designs" to build new weapons.