China has hardened its public posture toward the Soviet Union in recent days following a period of increased contacts and signs of a gradual improvement in relations.

The latest sign of a tougher stance came in today's edition of the official weekly magazine Peking Review, which gave prominent play to an interview with Vice Foreign Minister Qian Qichen. Qian said that while Sino-Soviet relations in the areas of economy, trade and technology have developed and official contacts are increasing, Sino-Soviet "political relations" have not improved.

In the interview, Qian, using what diplomats here characterized as unusually harsh language, accused the Soviets of repeatedly expressing the desire to improve relations with China while trying to "dodge" discussion of ways to remove what the Chinese call the three major obstacles to improved political relations between the two Communist nations.

The three "obstacles" are Soviet support for the Vietnamese occupation of Cambodia, Soviet involvement in Afghanistan and the Soviet troop presence in Mongolia and along the Sino-Soviet border.

Diplomats said that the harder Chinese line might have three aims:

*To reassure Thailand of support against Soviet-backed Vietnam at a time when the Vietnamese are on the offensive in Cambodia;

*To reassure Washington that there was no basic change in Sino-Soviet relations at a time when the transfer of U.S. technology to China is supposed to increase;

*To put pressure on the Soviets to make at least one gesture toward resolving at least one of the three obstacles.

The Soviets, said Qian, "agree to talk about the easing of international tensions and the elimination of regional conflicts, but at the same time show an unwillingness to talk about the Afghanistan and Cambodia issues.

"It seems the Soviet side has a misconception that it can get around these obstacles, or that the obstacles will vanish by themselves," said Qian. "This calculation of the Soviets is unrealistic and unwise," he said.

Qian is the Foreign Ministry official who has been conducting the normalization talks that the two sides have held in recent years at six-month intervals. The next round is expected to be held in Moscow in April.

Diplomats from both Western and Eastern European nations have been left guessing as to what the tougher Chinese statements mean. The best guess among diplomats here seems to be that the Chinese have several reasons for adopting a harder public line but that a slow improvement in Sino-Soviet relations nonetheless is likely to continue during the coming year.

Diplomats said the Chinese also are indicating that the talks held here early last month by Soviet Vice Foreign Minister Mikhail Kapitsa went less well than Soviet diplomats said they did. Kapitsa told reporters Dec. 13 that the two countries still have problems but have agreed to increase contacts, including setting a timetable for an exchange of visits by foreign ministers this year.

The Soviet diplomat said the talks with Qian and with Foreign Minster Wu Xueqian were "very nice, businesslike." At the time, Qian said there was still "no change" in Sino-Soviet relations. On Dec. 26, during a visit to Thailand, Chinese Foreign Minister Wu said a normalization of relations between China and the Soviet Union could not be realized if the Soviets refused to do anything to remove the three obstacles, especially the major obstacle of Soviet support for the Vietnamese occupation of Cambodia.

"China never allies itself or establishes strategic relationships with any big power," said Qian in the interview published in the English-language magazine. "It opposes all forms of hegemonism . . . . The two superpowers, the United States and the Soviet Union, threaten world peace with their arms race and contests in some of the 'hot spots' of the world."

"The Russians want us to believe that relations with the Chinese are moving forward," said a western diplomat here today. "The Chinese are telling us it's not the case." The same diplomat said Kapitsa's recent talks here had been "very difficult" and that the Chinese raised the issue of medium-range Soviet nuclear missiles aimed at China.

An Eastern European diplomat agreed that Kapitsa's talks "didn't go well" despite what some other Eastern European diplomats had said at the time. But he predicted that over the long run, China would continue to improve its ties with the Soviet Union even to the point of reestablishing relations between the two nations' Communist parties. This diplomat said that the Soviet leader, Mikhail Gorbachev, is "serious about improving relations" and he added that progress was possible on the Afghanistan issue.