Moscow and Peking, having sweetened the atmosphere of their relations, are seeking an agreed starting point for talking about major substantive policy differences.
A senior Soviet official, speaking to the Monitor March 14 amid the second round of Sino-Soviet political talks here, voiced optimism about future relations between the giant communist neighbor-states.
But, while listing what he termed significant improvements in atmosphere between Moscow and Peking, he and other Soviet officials said that to foresee a quick and easy reconciliation after the two-decade Sino-Soviet estrangement would be unrealistic.
''Many complex issues remain,'' the official said. A colleague added: ''Such issues don't simply vanish into thin air.''
The first official did say Moscow had discerned, in recent contacts with the Chinese, a similar ''realism'' on Peking's part. ''They (the Chinese) agree we must tackle issues carefully and effectively, one at a time.''
Initially, the Chinese had suggested that improved ties must depend on Soviet concessions on a package of issues: a troop withdrawal from Afghanistan, a withdrawal of support for the Vietnamese troop presence in Kampuchea, and a thinning of Soviet forces in Mongolia and on the Sino-Soviet frontier.
But even if such demands - ''obstacles, I would call them,'' said the Soviet official - ''are to be tackled one at a time, it remains to be agreed where best to start.''
The Soviets, said the official, are in favor of concentrating on ''purely bilateral'' questions such as troop levels and territorial disputes on their common border.
The official did not make clear whether the gap had been narrowed appreciably in the second round of talks - which began at the start of March and are expected by foreign diplomats here to wind up by the end of the month.
Yet several Soviet officials interviewed did stress what they portrayed as increasing signs of atmospheric improvement between Moscow and Peking. One official, a member of the Soviet Communist Party Central Committee, argued that ''a normalization in the atmosphere of relations has already been achieved.''
Among the signs cited:
* A recent visit to Peking by Alexander Bovin, a political commentator on the government newspaper, Izvestia. Mr. Bovin is said by colleagues to enjoy the personal confidence of Kremlin leader Yuri Andropov. One senior official, asked whether Mr. Bovin had carried a formal message from the Soviet leader to Peking, replied: ''No. Bovin himself was the message'' of commitment to seeking improved ties.
* A 40 percent growth in Sino-Soviet trade, although the previous level was so paltry that the level is nowhere near that of Chinese-US commerce.
* ''Less shouting at each other in the press,'' as a Soviet official put it. Indeed, various Soviet publications have begun to cover Chinese literature, art, and the like after a hiatus of many years.
* A number of exchanges involving academic, cultural, or sports groups. The most recent development on this front is an invitation for Soviet chess players to compete in a Peking competition in April. Soviet sources add that China, for the first time, has also expressed interest in participating in Moscow's international book fair, to be held later this year.