The United States plans to send a diplomat to the Cairo talks on the Middle East, but has held off an announcement while seeking to avoid a polarizing split in the Arab world, administration officials said yesterday.

The administration may officially announce today that it is joining the talks, called by Egyptian President Anwar Sadat to start as early as this weekend.

In Jerusalem, Prime Minister Menahem Begin announced that two senior aides would lead the Israel delegation to the Cairo talks. [Page A10]

Sadat has described the discussions as preparations for the Geneva conference. Administration sources said they needed time to "try to make this development as constructive as possible." It was necessary, these sources said, to try to assure other Arab naions, and the Soviet Union, that the United States was still committed to a "comprehensive peace settlement," rather than a maneuver to support a separate Egyptian-Israeli settlement.

Nevertheless, only Israel so far has said it will attend the Cairo talks, and it currently appears that the participants would be limited to Egypt, Israel and the United States.

An early U.S. response to Sadat's official announcement of last Saturday was held off yesterday as President Carter conferred through the day with his senior advisers. Between the White House meetings, Secretary of State Cyrus R. Vance met with Soviet Ambassador Anatoliy F. Dobrynin.

Although the United States and the Soviet Union are official co-chairmen of a reconvened Geneva conference, their strategies have been overtaken by Sadat's latest initiative.

The Soviet Union is a supporter of the militant Arab nations, including Syria, Iraq and Libya, which have condemned Sadat's dramatic overtures to Israel. Soviet participation in the Cairo meeting is therefore most improbable. The United States, however, wants to try to prevent the Cairo meeting from washing out all prospects for a broad-based Geneva conference.

There is special American interest in attempting to hold open the prospect for Syrian participation in a Geneva conference, along with Egypt, Jordanand Lebanon. Administration officials said yesterday that they were somewhat encouraged that Syrian President Hafez Assad, though he has assailed Sadat's trip to Israel, did not foreclose Syrian attendance at a Geneva conference, during a press conference yesterday.

The American participant in the Cairo talks that Sadat is organizing will be at a secondary level, possibly Alfred L. Atherton Jr., assistant secretary of state for the Middle East. Israel also is sending second-level officials. But precisely what will take place in Cairo is very unclear.

A blur of unusual uncertainty now extends over the whole Middle Eastern diplomatic scene.

White House press secretary Jody Powell said yesterday the United States is now pursing "a relatively low profile" in the Middle East.

"Obviously the position of this country has changed somewhat" as a result of the momentum generated by Sadat, Powell told United Press International "Our role changes from one of being a point man to supporting others."

Both Powell and State Department spokesman Hodding Carter III all but worte off the American-Soviet goal of a December date for a Geneva conference. Said Powell: "If the momentum is kept going, it may not be important whether the Geneva conference is under way before January."

The State Department's Carter said, "While that [a conference by December] has always been our goal, there are now clear problems that exist . . . I would simply observe what the date is today."

The President met in early morning with Secretary Vance and presidential national security adviser Zbigniew Brezezinski on the Middle East situation. Vance then conferred for about 40 minutes with Soviet Ambassador Dobrynin. Afterward, the President, Vance, Brezeinski and Vice President Mondale met for about an hour in early afternoon.

Powell and the State Department's Carter both denied that the administration was chagrined or discomfited by Sadat's call for Cairo talks. But there were qualifications in their descriptions of it. They said "such a meeting could be a helpful step in preparing for a Geneva conference."

"What is also at play here," said the State Department spokesman, "is the reaction of other parties . . . We do think it is necessary to get a really accurate summary of where they stand."

Criticisms of the administration's delay in responding to Sadat were voiced by Sens. George McGovern (D.S.D.) and Henry M. Jackson (D-Wash.), Associated Press reported.

"We ought to have been the first country to respond affirmatively," McGovern said. Jackson said, "We shouldn't be equlvocating. The more we equivocate, the more it weakens Sadat's hand . . . I don't understand why we haven't responded."

However, Sen James Abourezk (D-S.D.), a supporter of the Arab viewpoint, said any agreement reached without Palestnian support "would endanger peace rather than enhance it." Abourezk said Sadat's trip to Israel was "quite a bold thing," but he said "there seems to be a tendency for Sadat to move without the PLO [Palestine Liberation Organization] and Syria."

From the opposite side, swift U.S. action to join the Cairo talks was urged on President Carter in messages from Naomi Levine, executive director of the American Jewish Congress, and Rabbi Joseph P. Sternstein, president of the Zionist Organization of America. Sternstein said "the United States should not let pride stand in the way of participation in the peace process . . ."

Staff writer Edward Walsh contributed to this report .