Three days of U.S.-Soviet diplomatic meetings began here yesterday on a positive note with the signing of an agreement aimed at reducing risk of accidental nuclear war and optimistic words from Soviet Foreign Minister Eduard Shevardnadze, who said "a summit is necessary" between President Reagan and Soviet leader Mikhail Gorbachev.

Shevardnadze handed Reagan an eight-page, double-spaced letter from Gorbachev that a senior administration official described as "straightforward, constructive" and "dealing with every aspect of the relationship." The letter contained no date for a proposed summit but emphasized that the Soviets seek progress on arms control and held out prospect of a summit if this occurs.

The agreement, which establishes "nuclear risk reduction centers" designed to provide exchange of military information, was signed at a sunny ceremony in the White House Rose Garden by Shevardnadze and Secretary of State George P. Shultz with Reagan looking on approvingly.

Officials of both countries have expressed hope this week's meetings could clear the way for a U.S.-Soviet intermediate-range nuclear forces (INF) treaty to eliminate medium- and short-range nuclear weapons in the arsenals of both superpowers. They also have said that an INF pact could be signed at a summit in Washington in late November.

"I would like to think that this small gulp of hope is a prelude to a quenching of the global thirst for peace and security," Shevardnadze said at the signing ceremony.

Reagan called the agreement, which establishes risk reduction centers in Washington and Moscow, "another practical step" toward reducing the chances of accidental war.

He said he looks forward to the day when he and Gorbachev "can sign even more historic agreements in our common search for peace."

Despite the general mood of optimism and good feeling, a State Department official pointed out that the detailed discussions of the contentious arms control issues were just beginning yesterday afternoon following the Rose Garden ceremony, a briefing by Shultz and a friendly lunch hosted by the president for the Soviet delegation at the White House.

Meanwhile, the Soviet news agency Tass, striking a discordant note in a dispatch from Moscow, called a portion of the draft U.S. INF treaty tabled Monday in Geneva "a significant step backwards on the road to a treaty." The Soviets objected to a U.S. plan to keep missile warheads in Europe until missiles and launchers are dismantled instead of bringing them back to their home countries within one year.

But U.S. officials said Shevardnadze was generally positive during a half-hour discussion with Reagan after the Rose Garden ceremony. The officials said Shevardnadze emphasized the importance of overcoming obstacles to reach an INF agreement and quoted him as saying, "We must pass over from verbal confrontation to constructive debate."

The U.S. officials also said that Shevardnadze emphasized, in three hours of discussions with Shultz yesterday morning and in his meeting with Reagan, that there was "momentum" for an INF treaty and said it appeared that the obstacles to a pact could be worked out. Shultz took much the same view, telling reporters about the remaining INF issues, "I think that, at least as I would see them, they're soluble."

Leaving the State Department after an afternoon round of talks with Shultz which lasted more than two hours, Shevardnadze said, "We've achieved greater understanding {on INF} but still issues remain." He added that "on the whole, there is a reciprocal desire" to move ahead "but desire is not enough."

The Reagan-Shevardnadze discussions earlier in the day, which continued through the hour-long lunch, touched on a range of issues, including arms control and human rights, the officials said.

Reagan raised "a number of individual human rights cases," a U.S. official said, and Shevardnadze replied that the Soviets were concerned about human rights and receptive to the U.S. position "but wanted more credit" for the steps taken under Gorbachev to release political dissidents and permit more Jews and others to emigrate.

A senior U.S. official was asked if the two countries were "closer" as a result of the morning's meetings. "If you define 'closer' as being closer to an INF agreement, it's fair to say 'yes,' " the official replied.

But the two sides appeared to be no closer on the knotty issue of Reagan's proposed antimissile system, the Strategic Defense Initiative (SDI).

The Soviet position, reiterated by Shevardnadze in the White House discussion, is that limits on SDI are a necessary condition for large-scale reductions in strategic offensive nuclear arms.

"Let it be clear that I'm not going to bargain away SDI," the senior official quoted Reagan as replying to Shevardnadze.

Shultz and Shevardnadze set up joint "working groups" on arms control, bilateral relations and human rights. The arms-control and bilateral-issues working groups planned to meet last night while Shultz, Shevardnadze and a few close aides took a Potomac River cruise aboard the yacht of the U.S. chief of naval operations. The human rights group is scheduled to meet this morning.

The noon ceremony in the Rose Garden was the most cordial display in Washington of U.S.-Soviet amity in the nearly seven years of the Reagan administration. Against a backdrop of U.S. and Soviet flags, members of the two delegations congratulated each other for the agreement as Reagan and Shevardnadze exchanged hopeful statements that were translated into one another's language.

In his briefing of reporters Shultz said that "nuclear testing is one {issue} where progress might be made" and said also that "the Soviets have made some moves in the area of chemical warfare lately that are promising."

Similar remarks were made by a Soviet official Monday, who said, "We are coming now to your position" on nuclear testing "not because we like your position but because we want to drag you into negotiations on a comprehensive test ban."

Staff writer R. Jeffrey Smith contributed to this report.