The United States and the Soviet Union agreed tonight to resume negotiations on reducing strategic nuclear arms and medium-range missiles in Europe and to begin a third set of negotiations, on "preventing an arms race in space."

The surprise result of two days of highly secretive talks between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko is expected to bring a return of full-scale weapons bargaining between the two nations by March, according to U.S. sources.

Shultz, in a press conference announcing the accord, said he did not agree to any preconditions such as a halt to U.S. antisatellite tests or other space development programs as desired by Moscow.

Although officially described as "new" in a face-saving gesture to the Soviet Union, the strategic and medium-range bargaining will, in effect, continue negotiations that the Kremlin broke off a little more than a year ago and vowed not to continue as long as NATO continued to deploy a new generation of missiles in Western Europe.

In return, the Soviets won U.S. agreement to create a forum in which President Reagan's Strategic Defense Initiative or "Star Wars" plan at least can be discussed on a systematic basis.

Only a tenuous link was cited between the two sets of offensive arms negotiations and the new space arms negotiations, which are to be considered by separate groups within what is technically one overall arms control delegation.

A joint statement said all three areas would be "considered and resolved" in relation to each other, but a member of the U.S. team here said the relationship is "very fuzzy." Shultz told reporters that the United States believes it should be possible to move ahead in some areas without waiting on the others.

The ultimate goal of both sides was set forth in their joint statement as "the complete elimination of nuclear arms everywhere." But U.S. officials conceded that already slow progress in strategic arms reduction will be complicated by the stalemate between the two countries on space weaponry. The negotiations on medium-range missiles become more involved as both sides deploy more of these weapons in Europe.

A Soviet spokesman cautioned that today's agreement did not mean a new age of detente was dawning, but he saw it as "a small crack in the East-West ice."

The agreements were reached in 14 1/2 hours of discussions here during the past two days by Shultz, Gromyko and a few aides on each side. U.S. participants said the opening positions of the two sides were so far apart that it seemed likely at first that it would take another meeting of the two foreign ministers at some other time and some other place to resolve the differences.

There were "conflicting signals" from the Soviet side yesterday, but this morning the ice began to thaw in the frigid winter of a Swiss cold wave. By this afternoon it became clear, according to these accounts, that Gromyko was ready to deal, especially in the space arms area that by all acounts was the most contentious.

Today's afternoon session began at 2:30 in a small paneled conference room in the six-story, fortress-like U.S. mission. It was due to be over at 5:30 but in fact lasted almost to 8 p.m. as Shultz and his key negotiatiors left the conference room repeatedly to confer with a large and diverse U.S. delegation caucusing nearby to consider language on the "subject and objectives" of the future negotiations as they were being hammered out by the two foreign ministers.

Shultz paid tribute tonight to National Security Adviser Robert C. McFarlane and other representatives of various government departments and factions that had often warred with each other in the past over arms policy, but who worked together here.

Richard Perle, the assistant secretary of defense who has been considered a key opponent of arms control negotiations and the terms on which they have been conducted, took part in the deliberations. Following tonight's announcement he said, "This is a good beginning, but we've got our work cut out for us."

No date for the beginning of the new three-part negotiations was agreed upon in today's meetings. But the two sides did announce that they will settle the date and the site through diplomatic channels within a month.

A U.S. official said Geneva is the likely site and that the negotiations are expected to begin in March.

Earlier it had been anticipated that Shultz would go to Moscow around March to work on arms control issues and broader political questions between the two governments. Shultz discouraged that speculation tonight, although he did say that he and Gromyko have discussed a further meeting. Since Gromyko was last in Washington, it would be Shultz's turn to go to the Soviet Union under normal diplomatic practice.

Shultz said that there had been no discussion between him and Gromyko of a summit meeting of Reagan and Soviet leader Konstantin U. Chernenko.

What today's arms accord may mean to the political relations of the two nuclear superpowers is far from clear. The resumption of negotiations does not guarantee progress in reducing strategic offensive arms or intermediate-range weapons in Europe, nor does the beginning of arms control talks about outer space mean that the deep divisions between the two governments in this area have been bridged or even narrowed.

Nevertheless, the fact of active arms control discussions between Washington and Moscow provides forums and opportunities to discuss differences, and the absence of such discussion during the past year contributed to an increase in tensions.

Soviet Foreign Ministry spokesman Vladimir Lomeiko came to U.S. Information Agency press headquarters at the Intercontinental Hotel, where Shultz had just completed a press conference, to discuss Moscow's viewpoint with reporters.

Lomeiko said he "wouldn't jump to a conclusion" that a new age of detente was starting because of the agreements reached today, but added that the very fact of the place and forum for his briefing signified a small crack in the East-West ice.

"The new year gave us a small but new chance" to do something about curbing the arms race, the Soviet spokesman said.

"It is necessary to abandon old thinking that more arms bring more security," he said. He added: "In the nuclear age there is an entirely new situation when enough arms have been accumulated to destroy everything."

In the first public Soviet comment on the talks today, analyst Valentin Zorin, broadcasting from Geneva shortly before the talks drew to a close, said on Soviet television that the meeting had given both sides an opportunity to clarify their positions, "a factor that facilitates the continuation of the Soviet-U.S. dialogue."

Zorin, however, also noted that the position of the U.S. delegation echoed "Washington's earlier ideas which make the search for a mutually acceptable agreement difficult," Washington Post correspondent Celestine Bohlen reported from Moscow.

Shultz is scheduled to leave Geneva for Washington early Wednesday and arrive home in time to brief Reagan before the president's 8 p.m. news conference.

McFarlane is heading to London to explain the developments to Prime Minister Margaret Thatcher and her government. Arms adviser Paul Nitze is going to Bonn to see West German Chancellor Helmut Kohl and other officials. Other members of the U.S. team are fanning out to brief allies as far away as Australia and New Zealand.

The key area of U.S. leverage and of U.S. compromise was outer- space weapons, which have brought forth vehement Soviet complaints since Reagan announced his Star Wars plan for intercepting and destroying enemy missiles on March 23, 1983.

Before leaving Washington, senior U.S. officials said the Reagan administration was willing to engage in discussions with the Soviets about the space-based defense program but not to agree to limitations or concessions.

Shultz refused to accept a Gromyko proposal that would have sharply limited testing and development of space weaponry, according to members of the U.S. delegation. He also refused to accept the Soviet proposal that space talks be aimed at "demilitarization" but accepted the objective of "preventing an arms race in space."

In addition, Shultz insisted and the Soviets evidently accepted that the space negotiations cover armaments based on Earth, such as ground-based antiballistic missile systems, as well as the futuristic space-based arms.

Members of the U.S. delegation said Shultz, as planned raised the question of a Soviet radar installation in Central Siberia which Washington believes to be a violation of the 1972 Anti-Ballistic Missile Treaty.

Shultz spoke in approving terms of the ABM treaty at a press conference tonight, saying that an aim of the forthcoming negotiations is to "reverse the erosion of the ABM treaty that has occurred over the last decade."

He intimated at the press conference and reportedly said clearly and explicitly to Gromyko in the negotiations that the United States does not believe it is violating the 1972 treaty and will not do so without further negotiations with the Soviets.

Today's joint announcement was the culmination of a process which began at least a year ago when Reagan made a speech calling for a resumption of U.S.-Soviet dialogue and negotiations.

The Soviets were reluctant to take major steps to improve relations at a time when Reagan was running for reelection. They did, however, propose talks on the demiliarization of space this past summer and send Gromyko to New York and Washington in September.

Only 11 days after Reagan's reelection, Chernenko sent a message to Washington agreeing in principle to new negotiations covering offensive and outer-space arms, and suggesting a meeting of Shultz and Gromyko in early January to decide on "the subject and objectives" for such bargaining.