The Soviet Union cautioned the West last night against supplying weapons to China, clearly suggesting that such a move would eliminate chances for a new Strategic Arms Limitation agreement with the United States.

A statement issued by the ruling Politburo said China was a "serious threat" to peace, that it is involved in "expansionist activities" and that it is trying "to gain access to NATO military arsenals."

The statement, underscoring Moscow's determination "to pursue the course of detente," was made after a meeting of the Politburo, the Soviet Union's supreme policy-making body, apparently devoted to the global antiSoviet foreign policy pursued by Peking's new leaders.

A statement of this kind from the Politburo is rare and generally indicates the issue is of deep concern to the Kremlin leadership.

A Chinese military delegation visited a number of NATO countries earlier this year looking at Western weapons. Subsequently Peking completed an agreement for French antitank missiles. The Chinese have also expressed interest in British transport planes and the West German Leopard tank as well as various Western satellite systems.

There are no indications that the United States is at this stage prepared to supply arms to Peking but the indications that China may acquire more Western military equipment and technology have fueled Moscow's worst fear, that is, having hostile and powerful forces with links to each other on both flanks.

Several events in recent weeks --especially the conclusion of a Sino-Japanese friendship treaty and Chinese Communist Party Chairman Hua Kuo-feng's recent unprecedented visit to Eastern Europe -- has heightened Soviet concern over Peking's challenge.

But the Politburo statement indicated that the fundamental concern is the posibility that the West may decide to provide China with arms and that this kind of cooperation would tilt the strategic and political balance against Moscow.

The linkage between China policy and SALT follows reports earlier this summer that Soviet and American negotiators have made progress at their nuclear arms talks in Geneva and that an accord could be reached before the end of the year.

"Major questions connected with curbing of the arms race are already the subject of negotiation," the statement said. "It is important that the work accomplished at the talks should not be annulled but (instead) be crowned with important results. Specifically this concerns Soviet-American negotiations on the limitation of strategic offensive weapons and the Vienna talks on military forces and arms reductions in central Europe.

"The Politburo holds that in the present situation it is especially necessary to resolutely counter any step that could undermine the process of detente and reverse the international development back to the cold war."

Although its tone was more restrained than other criticisms of China published in recent months, the statement said:

"The Politburo underscores the serious threat presented to the cause of peace and socialism by the action of the current leaders of China. In pursuing their great-power, hegemonie course, Peking openly places stake on the increase of international tension and is using all means to undermine the position of the socialist community.

"Trying to gain access to NATO military arsenals, the rulers of China in various ways are advertising their hostility toward the Soviet Union and other socialist countries and are working for an uncontrollable arms race."

The statement charged the Chinese "already today" are involved in "direct expansionsist action" and said that this was evident in Peking's pressure on Vietnam.