West European leaders welcomed Mikhail S. Gorbachev's pledge to unilaterally cut Soviet conventional forces, but some warned that even after such a move the Soviets will remain militarily superior to NATO.

China's leaders also welcomed the Soviet president's pledge, in a speech at the United Nations in New York on Wednesday, to demobilize 500,000 troops.Gorbachev said that as part of the initiative to reduce the Soviet army to about 5 million soldiers "a major portion of Soviet troops temporarily stationed (in Mongolia) will return home."

"This is a positive development," Foreign Ministry spokesman Li Zhaoxing said in Beijing.

China long has demanded the Kremlin reduce its troop presence along their common border as a condition for normalizing relations after nearly 30 years of estrangement. The Soviet Union stations nearly 50,000 troops in Mongolia and pulled out about 12,000 soldiers in 1987.

Most East European news media reported without comment on Gorbachev's promise, including a pledge to remove 5,000 tanks and 50,000 troops from the German Democratic Republic, Czechoslovakia and Hungary.

But the Czechoslovak Communist daily Rude Pravo said the aim "is to give conventional disarmament a vital shot in the arm."

"The point is literally to force Western partners to stop giving speeches . . . about the need for disarmament, about their concern about the buildup of troops and military technology in Europe, and to start acting," said foreign editor Milan Jelinek.

State TV in communist Poland carried without comment footage of Gorbachev making the announcement.

In Warsaw, Jacek Czaputowicz, co-founder of Poland's pacifist group Peace and Freedom, said it could be the "first step" to withdrawing Soviet troops from Poland.

Prime Minister Margaret Thatcher of Britain called it "a very positive step forward" and "just exactly what we wanted."

Asked if she thought the West should respond with its own proposals, Thatcher said: "According to the briefing I had . . . it reduces their superiority in conventional weapons from about 3-to-1 to 2-to-1. So it is a very, very fundamental change that he is making."

In Brussels, NATO Secretary-General Manfred Woerner said the move "seems to be a step in the right direction" but warned big imbalances would remain.

West German Defense Minister Rupert Scholz said Gorbachev must follow up with "actual and verifiable withdrawal of troops and reduction of forces."

Defense Minister Jean-Pierre Chevenement of France told reporters that Gorbachev's "art is to make announcements before he is forced to."