MOSCOW, AUG. 25 -- Senior Soviet diplomats today warned that talks scheduled to take place between Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze in mid-September are still jeopardized by a lingering dispute over 72 Pershing IA missiles that carry American nuclear warheads.

But after two days of preparatory discussions here for the upcoming Washington meeting, Assistant Secretary of State Richard Schifter gave a more upbeat assessment. In an interview, he said the meeting between Shultz and Shevardnadze is still on track and U.S. planners have received no other signals from their Soviet counterparts.

"The word that I saw just this morning from Washington is that the Shultz-Shevardnadze meeting is on," Schifter said. "As far as we're concerned we know of no conditions for it."

The meeting, due to be held from Sept. 15 to 17 in Washington, is viewed on both sides as a key preparatory session for a proposed summit later this year between Kremlin leader Mikhail Gorbachev and President Reagan. If the meeting of ministers is canceled, the summit would also be cast into doubt, diplomats here said.

Gorbachev today proposed a meeting of the leaders of the five members of the United Nations Security Council, a meeting that is viewed here as a possible alternative to a third Reagan-Gorbachev summit. Such a meeting could take place at U.N. headquarters in New York when the assembly opens in late September, western diplomats here said.

"It would be useful to discuss problems of disarmament and development in terms of fundamentals at a special meeting of the top leaders of the United Nations Security Council member states," Gorbachev said in a message released by the Soviet news agency Tass. The message was sent to participants of an international disarmament and development conference meeting here.

{In Washington, the State Department rejected the Soviet proposal, saying the issues of disarmament and economic development in the Third World are not "directly or functionally interrelated."}

A dispute between U.S. and Soviet arms negotiaters in Geneva over the 72 Pershing IA missiles based in West Germany "may prove to be the obstacle that will make it impossible to organize a fruitful meeting" between Shevardnadze and Shultz, arms control specialist Victor Karpov said in an interview released today by the Tass.

Soviet Foreign Ministry spokesman Gennadi Gerasimov echoed the assessment in a press briefing. Charging U.S. negotiators in Geneva with avoiding a discussion of this major issue, Gerasimov said the Geneva talks are "crucial" and "the meeting of the two ministers will depend on that."

The differing assessments of the preparatory talks suggest that the Soviet Union is seeking to put pressure on U.S. negotiators in Geneva to include the 72 West German Pershing missiles and their U.S. nuclear warheads in a treaty to scrap all medium- and short-range missiles on both sides, according to some western diplomats here.

The diplomats said the conflicting assessments also reflect the two sides' differing interests in the talks.

The United States favors high-level talks that cover arms control, human rights and regional and bilateral issues, Schifter said. The Soviets are putting the heaviest stress on the need for progress in arms control.

Schifter, who oversees human rights issues in the State Department, today ended two days of talks with Soviet officials on the human rights component of the Shevardnadze-Shultz meeting.

Schifter said the U.S. side has focused on the issues of spouses divided between the United States and the Soviet Union, Soviet political prisoners and persons who are incarcerated on religious grounds, the official Soviet use of psychiatry for the purposes of political repression and the need for increased Soviet emigration to the West.

He said he pressed for specific cases of Soviet dissidents to be allowed to emigrate, but declined to identify them.