UNITED NATIONS, SEPT. 26 -- North Korea has demobilized more than 30,000 troops in the past two months as a unilateral gesture to ease tension on the divided peninsula and is prepared for much deeper military reductions in the context of negotiations with South Korea and the United States, a senior North Korean official said today.

Kang Sok Ju, North Korea's deputy foreign minister and chief of its observer delegation to the current session of the U.N. General Assembly, said in an interview that some of the demobilized men are helping to build a 150,000-seat stadium in the capital of Pyongyang, which North Korea still hopes will be the site of some events in next year's Olympic Games.

North Korea announced July 23 it would unilaterally reduce its forces by 100,000 men before the end of this year, and Kang said the 30,000 to 40,000 demobilized so far signify that the target will be achieved. As part of the same declaration, North Korea proposed negotiations involving South Korea and the United States next March aimed at reducing North and South Korean forces by stages to 100,000 on each side by 1991, and removing U.S. troops and nuclear weapons.

Kang said the military reduction of 100,000 men currently under way would leave his country's armed forces at 420,000 men. However, the most recent U.S. official estimate, made before the reductions were announced, put North Korea's armed forces at 830,000.

The divided peninsula, which is one of the world's most heavily armed areas of confrontation, also includes South Korean forces estimated at about 600,000 and U.S. forces of 40,000 armed with nuclear weapons and other highly sophisticated arms.

South Korea and the United States treated the recent North Korean troop announcements and proposals with skepticism, and Washington said that any mutual cuts should be negotiated between the two Koreas without U.S. participation.

According to material provided by Kang, North Korea this year has made an unprecedentedly large number of public statements and proposals regarding north-south talks or military reductions. Many of these drew counterproposals or other responses from South Korea, but so far diplomatic and other talks between north and south remain dormant.

"We gave the ball to the South Korean side but they did not return the ball," charged Kang. "Instead, they returned a stone."

Reminded that South Korea is going through a period of intense internal political activity leading to the popular election of a new president in December, Kang said a decision to pursue disarmament talks with the north would contribute to stability in the south.

"We are following developments in the south with deep interest," he said. "It is our hope that a democratic government will be set up in the south and we hope the election will be held in a fair way. We think the establishment of a democratic government in South Korea would contribute to an improvement in the relationship between north and south."

Kang said he is prepared to have "contact" with South Korean officials during his current trip to New York, but that this is different from the official talks the north has proposed.

Regarding U.S. policy, Kang expressed frustration with the results of a shift announced by Washington in March permitting U.S. diplomats to conduct conversations with North Korean officials at diplomatic receptions and other "neutral" locations.

"The idea was good but we have just exchanged greetings, talk about the weather, food and such," Kang said. "If we attempt to discuss something of a political nature, the Americans are reluctant to talk . . . . Just contacts at social functions cannot solve our problems."

Kang said North Korea is "surprised" and "disappointed" at recent U.S. statements suggesting an increase in U.S. air and naval deployments around South Korea before and during next September's Seoul Olympics in order to deter any interference from the north. North Korea fears this is a "pretext" for more militarization, he said. "We have no intention to take any action" to interfere in the games, he said.

Kang said North Korea still hopes to host some Olympic events, a possibility that has been under negotiation with the south and the International Olympics Committee. He said the failure of Pyongyang to participate might contribute to a "complicated" environment, including unwillingness of some nonaligned countries to participate in Seoul. He seemed virtually to concede that the Soviet Union and Chinese athletes are likely to be in Seoul, saying that "we don't care" whether they participate.