U.N. Secretary General Kurt Waldheim, with the apparent approval of the United States and the Soviet Union, issued a call for a new Middle East conference yesterday that may overshadow Egyptian President Anwar Sadat's plan for talks in Cairo.

Waldheim told reporters at the United Nations that his conference, which like Sadat's is designed to prepare the way for a resumption of the Geneva peace talks, could be held "within a couple of weeks" of the Cairo meeting.

It is now apparent, Waldheim said, that the Cairo conference will have "only limited participation." He was referring to the fact that the Soviet Union, Syria, Jordan, Lebanon and the Palestine Liberation Organization and spurning Sadat's invitation to send representatives to Cairo to prepare for a Geneva conference on a Middle East settlement.

"I feel that if we want a Geneva conference, we must have all the parties concerned," Waldheim said.

The United States officially announced yesterday that it will send a representatives to Cairo "at the expert level" when a date for the meeting is set.

Waldheim also announced that he will send a representatives to the Cairo talks. Gen. Ensio Siilasvuo of Finland, Jerusalem-based chief coordinator of the U.N. peacekeeping force in the Middle East.

Both of these announcements, however, were overshadowed by what now appears to be a diplomatic surge to accommodate the opponents of Sadat's initiative, including the Soviet Union and the militant Arab nations.

Only the United States, Israel, and the U.N.'s token representative, Gen. Siilasvuo, are now slated to join Egypt in the Cairo talks.

Waldheim's announcement aroused immediate suspicion that it was a preemptive move against Sadat's strategy, supported by the United States and the Soviet Union. Carter administration officials strongly denied that.

Waldheim said he discussed the proposal in general, but not in detail, with the United States and the Soviet Union. Secretary of State Cyrus R. Vance discussed the plan with Waldheim, administration officials acknowledged, but said: "It was not our idea and we did not initiate it."

U.N. sources said Waldheim would never have surfaced the proposal if it had been opposed by either the United States or the Soviet Union. "Nobody threw cold water on it," one U.N. official said.

Officially, the United States limited itself to a neutral comment on the Waldheim move. "We will take his suggestion under consideration and see what his consultations on the idea might produce," said a State Department spokesman.

The Soviet Union formally notified the United States and the United Nations yesterday that it will not take part in the Cairo talks.

In Moscow, Soviet Foreign Minister Andrei A. Gromyko officially voiced disapproval of Sadat's initiatives - the Kremlin's first direct comment on Sadat's pathbreaking trip in Israel and his call for a pre-Geneva conference in Cairo.

Speaking at a reception for Syrian Foreign Minister Abdel Halim Khaddam, who is visiting Moscow to confer on strategy to counter Sadat, Gromyko said:

"We ourselves are systematically working to promote the solution of these problems. But if one country demonstratively departs from the common Arab front and sacrifices the interests of the Arab states as a whole, first of all those who have suffered from Israeli aggression, this then is quite another matter."

"How can one approve such actions?" Gromyko asked rhetorically. "One cannot."

Gromyko said Sadat's actions had put the Geneva conference on the Middle East "in a more difficult situation than before.

"Let us wait and see how the current negotiations and consultations conclude," he said.

Opposition to Sadat and his overtures to Israel is now spread across a broad front.

Libya's official news agency said yesterday that a summit meeting of the Arab opponents of Sadat's initiatives will be held as scheduled on Thursday, in Tripoli.

Some uncertainty had been raised about that meeting because Iraq also has called for an Arab summit meeting in Baghdad next week for "mobilizing (Arab) revolutionary forces" against Sadat's plans.

Particpants in both the Tripoli and Baghdad meetings are expected to include Syria, Libya, Iraq, South Yemen, Algeria and the Palestine Liberation Organization (PLO).

American attitudes, particularly in Congress, run in the opposite direction. Congress, without dissent in either House, approved a resolution yesterday commending Sadat and Israeli Prime Minister Menahem Begin "for the courageous steps they have taken to resolve the differences between their nations, and to bring peace between Israel and her Arab neighbors through face-to-face negotiations in the context of a Geneva conference."

Sadat proposed the Cairo talks in hopes of achieving enough agreement in advance of a Geneva conference to avoid blocking moves that may be made there by militant Arab nations or the Soviet Union, which supports them. The Egyptian president's own relations with Moscow are openly strained, as Gromyko's remarks abundantly underscored.

State Department spokesman Hodding Carter III said the United States will name it representatives to the Cairo talks at a later date.