China and the Soviet Union agreed today to exchange visits of foreign ministers, in what western diplomats called another important step toward reducing tensions and normalizing relations.

It would be the first time in more than two decades that the two sides have had such an exchange.

Soviet Vice Foreign Minister Leonid Ilichev, who arrived here today for another round of talks on normalizing relations between the two countries, told reporters here that an exchange of foreign ministers' visits had been agreed upon but that details still had to be worked out.

These normalization talks began in 1982 on a semiannual basis. On Friday, Ilichev, the Soviet Union's leading China specialist, is to meet with his counterpart, Chinese Vice Foreign Minister Qian Qichen, to begin the talks' seventh round.

The Chinese have stated that there are three major obstacles to a normalization of Sino-Soviet political relations, including the stationing of Soviet troops along the Chinese border and in Outer Mongolia, the presence of Soviet troops in Afghanistan, and Soviet support for Vietnam's occupation of Cambodia. But these issues now seem not to be hindering the normalization process.

"The Chinese are very unconvincing when they say there has been no progress in political relations," said a western diplomat. "Political progress means more dialogue and a reduction in tensions, and that's exactly what's happening." The agreement on foreign ministers' visits, he said, represents "another symbolic step forward."

"The Chinese have said there can be no political normalization until there is movement on the three obstacles," said another western diplomat. "But the fact remains that they are doing everything which two major states need to do in order to have relations with each other."

Chinese Foreign Minister Wu Xueqian met with Soviet Foreign Minister Eduard Shevardnadze in New York on Sept. 26 in what the official New China News Agency described as a "frank and friendly atmosphere." According to the Chinese news agency, Shevardnadze invited Wu to visit the Soviet Union at an undetermined time, and Wu invited Shevardnadze to visit China at an "appropriate time."

The last time a Soviet foreign minister went to Peking for talks was in 1959.

The Soviets and Chinese were allies in the 1950s but split in a conflict over disputed territory and ideological and foreign policy issues. Western diplomats doubt that the old alliance between the two Communist giants can ever be restored, and the Chinese have said it will not be. But the two nations have been gradually expanding their economic, cultural and sports ties as well as student exchanges.

In July, the two nations signed a five-year, $14 billion trade agreement as well as a cooperation accord under which the Soviets are to help modernize a number of Chinese factories that had been built with Soviet assistance in the 1950s.