WASHINGTON -- The United States and the Soviet Union, in a step toward a 1986 summit, announced yesterday that planning talks will be held here in September between Secretary of State George P. Shultz and the Soviet foreign minister.

No date has been set for the long-delayed summit itself, Charles E. Redman, a State Department spokesman, said in announcing the talks Sept. 19-20 between Shultz and Eduard A. Shevardnadze.

However, another U.S. official, who spoke only on condition of not being quoted by name, said "in the sense that we're going to take these steps I am more optimistic" of a summit by year's end.

The official said Shevardnadze probably would meet with President Reagan, as well. But at the White House, a spokesman, Dan Howard, said "there are no such plans at the present time."

Summit preparations were slowed by a slump in U.S.-Soviet relations after Reagan and Soviet leader Mikhail S. Gorbachev held their "fireside" summit last November in Geneva.

But in a recent exchange of letters, Reagan and Gorbachev asserted their determination to reduce sharply U.S. and Soviet nuclear weapons-not-withstanding an apparent conflict in their approahces.

U.S. and Soviet delegations also have held meetings in Geneva on nuclear weapons tests and on the two strategic arms limitation treaties that Reagan intends to scrap. Separate talks to slow the spread of nuclear technology to other nations were held in Moscow.

Reagan and Gorbachev had agreed on a 1986 summit in Washington and a 1987 summit in Moscow. In fact, the Soviet leader had been expected here in late June or July.

But the U.S. bombing of Libya in April, in what was described as retaliation for terrorism, prompted the Soviets to cancel a Shevardnadze visit in May. Subsequently, Reagan announced he no longer would observe the unratified 1979 SALT II treaty, which imposed ceilings on various U.S. and Soviet long-range nuclear weapons.

Last week, a U.S. delegation in Geneva informed the Soviets that the 1972 SALT I treaty, which set interim constraints, also would be abandoned.

Reagan and Gorbachev have asserted, however, that nuclear weapons on both sides must be curbed. Negotiations are due to resume in the Swiss city in September, dealing also with the U.S. anti-missile research program, known commonly as "Star Wars."

Redman said Shultz and Shevardnadze are expected to "review progress achieved in areas addressed by President Reagan and General Secretary Gorbachev in their November 1985 meeting and discuss what additional preparations may be needed for a summit meeting between the two leaders later this year."

The spokesman said the Shultz-Shevardnadze talks also would range over "all areas" of the U.S.-Soviet relationship.

The earliest practical date for a summit meeting is late November since Reagan has ruled out a Gorbachev visit during the U.S. congressional election campaign.

The agenda is likely to cover four broad areas: arms control, regional conflicts, U.S.-Soviet relations and human rights.

The Soviets want to concentrate on the nuclear weapons issue, while the Reagan administration sees arms control as an important topic but not one that should dominate the next Reagan-Gorbachev meeting.

At the same time, the Soviets are reluctant to place special emphasis on human rights, which is the West's way of referring to the the treatment of Soviet dissidents, minorities and other citizens.

Looking for a pre-summit compromise, the U.S. official who demanded anonymity, said "there's a willingness on the part of both parties to discuss each other's agenda" at the Shultz-Shevardnadze talks.