MOSCOW, OCT. 29 -- A Soviet Foreign Ministry spokesman said today that the United States and the Soviet Union have agreed on the terms of a summit meeting to take place before the end of the year.

Boris Pyadyshev, in a shift of terminology, told a press briefing that the two sides have reached "an understanding that together with an agreement on medium- and shorter-range weapons, questions relating to strategic offensive weapons and the {Anti-Ballistic Missile} treaty will be thoroughly discussed."

"This will be sufficient to hold a summit meeting this year," he said.

Last week during a visit here by Secretary of State George P. Shultz, the Soviets had insisted on reaching agreements on "key provisions" on strategic and space defense issues before setting the dates for a third summit meeting between Soviet leader Mikhail Gorbachev and President Reagan.

{The Associated Press quoted an unidentified Soviet official as saying the Kremlin proposes that the meeting take place during the first week in December.}

Pyadyshev, first deputy director of the Foreign Ministry information department, made the comments as Foreign Minister Eduard Shevardnadze, in Prague for a meeting with Warsaw Pact foreign ministers, headed to Washington for a two-day visit to speed along an agreement on a summit. Pyadyshev said he did "not rule out the possibility" that dates for the Reagan-Gorbachev meeting would be announced this weekend.

{Shevardnadze, traveling with a small group of top aides including Viktor Karpov and Alexander Bessmertnykh, arrived in Washington early Friday with a letter for Reagan from Gorbachev. He told reporters at Andrews Air Force Base that a summit "is a possibility. . . . We are in a fairly good mood, and we have a good basis (for discussions).}

In Prague, Warsaw Pact foreign ministers endorsed Moscow's agenda for a Soviet-U.S. summit meeting. The Soviet news agency Tass said that, in its communique, the Warsaw Pact expressed hope that an agreement to eliminate medium- and shorter-range nuclear missiles could be signed "shortly."

The communique added, in stronger language than Pyadyshev used, that it was also "necessary . . . to put on record an accord on the main provisions of the future agreements on strategic offensive arms and the nondeployment of weapons in outer space."

The Warsaw Pact ministers said an agreement on intermediate-range nuclear forces (INF) and "the attainment of accords" on the other arms control areas would "genuinely lay the beginning for the process of nuclear disarmament," Tass reported.

Pyadyshev insisted the new optimism about the summit was the result of a two-sided "advance in the Soviet-American dialogue." He said Shultz's two-day visit to Moscow last week had been too short to resolve all outstanding issues.

"We did not manage to deal with all the questions in the necessary depth," said Pyadyshev. "A pause was needed." He noted that at a meeting between Shevardnadze and U.S. Ambassador Jack Matlock here Tuesday, "both the American and the Soviet sides had something additional to say."

By now calling for a "discussion" instead of an "agreement" on "key provisions" on the combined issue of cuts in strategic weapons and observance of the ABM treaty, the Soviet Union seems to have shifted the timetable it had been seeking on arms control.

Originally, Moscow was pushing for an interim accord on cuts in strategic arms reduction talks (START) this fall, in addition to the INF treaty, with a full START treaty expected by mid-1988. In the Soviet view, strategic cuts are dependent on the United States agreeing to restrictions on its Strategic Defense Initiative program for a space-based antimissile system.

Today, Pyadyshev showed new flexibility, saying the discussion on START at the coming summit "may have a varying degree of intensity and bring about various results."

"We would consider this discussion successful should an interim agreement be worked out in the form of key provisions, or directives for the delegations" at Geneva, he said.

Washington Post correspondent Jackson Diehl in Warsaw contributed to this report.