Saudi Arabia's King Khalid, on the eve of a hotly contested U.S. Senate vote, yesterday appealed to the United States to supply his country with sophisticated F15 jet fighters as an urgent matter because of "recently stepped-up communist expansion in the area."

In a letter to President Carter, released by the State Department amid a flurry of 11th-hour lobbying, Khalid emphasized that "the planes are being acquired for defense." He said Saudi Arabia's "long and increasingly close relationship with the United States is, even with all of its proven mutual benefits, still at only an early stage of reciprocal worth." He did not elaborate.

Khalid did not describe in his letter the "stepped-up" communist activity, but on other occasions Saudi officials have expressed concern about Soviet and Cuban forces in Ethiopia across the Red Sea from Saudi Arabia, and a reported Soviet-Cuban buildup in South Yemen on Saudi Arabia's southern border.

Carter, in a meeting with Hispanic editors at the White House Friday, condemned "unnecessary and excessive use of military forces" by Cuba in South Yemen as well as in Africa. The State Department, in a statement to reporters, estimated yesterday that more than 1,000 "foreign communist personnel" are in South Yemen, at least half in civilian advisory capacities.

The statement said "as many as 500 to 600 Cubans" may be present, some as civil advisers and others training paramilitary forces. Soviets and East Germans also were reported present. The statement added that Cuban elements have been in South Yemen since the early 1970s but that their presence was augmented during the Soviet-Cuban buildup in Ethiopia. South Yenmen was a staging area for that operation.

The Saudi letter was sent as Senate Majority Leader Robert C. Byrd (D-W. Va.) confidently predicted the Senate will approve the jet sale on Monday. A failure to do so, he told a press conference, "would be a victory for violence, revolution and the clients of the Soviet Union."

The proposed sale of 60 F15 fighters to Saudi Arabia, 50 F5E fighter-bombers to Egypt and 15 F15s and 75 F16 fighter-bombers to Israel has brought on a major test in Congress of the Carter administration's Middle East policies. With high stakes involved, the White House and the State Department are reported to be heavily involved in lobbying uncommitted senators this weekend, while Jewish leaders and congressional supporters of Israel are reported to be in a lastditch lobbying campaign on the other side.

Byrd said he is determined that the matter will come to a vote Monday after no more than 10 hours of debate.

He refused to speculate on what the actual vote will be, but appeared confident he had the votes to allow the sales package to proceed.

His ranking assistant in the Democratic leadership, Sen. Alan Cranston (Calif.), said he counted 40 senators in favor of the sales, 35 against and the views of 25 unknown.

Cranston opposes the sale, saying it is senseless to supply 200 sophisticated fighter planes to an area already struggling to achieve peace.

Byrd predicted the undecided senators will divide almost evenly - and the antisale forces would not muster enough support to win.

"I think we have the votes to allow the sales to go forward," he said.

But he repeatedly tried to reassure reporters that the vote should not be considered a defeat for Israeli forces.

"It doesn't indicate any lessening in support for Israel. It is in the interest of Israel," he said, adding later, "It is a victory for Israel."

The Senate Foreign Relations Committee deadlocked 8 to 8 on the sales issue Thursday and sent the matter to the full Senate without a recommendation.

Byrd said he will attempt to avoid a direct up-or-down vote on the package by making a motion to table or indefinitely postpone further consdieration of all efforts to block the sale after the debate.

A vote to table would have the practical effect of allowing the sales to go ahead.

In an 11th-hour flurry, the White House yesterday sought to rebut suggestions that Saudi Foreign Minister Prince Saud Faisal, in a television interview with Walter Cronkite, had repudiated some of the limitations announced by the United States on the Saudis' use of the advanced warplanes.

In a CBS interview broadcast Friday night, Saud said his government would accept any limitation of warplane deployment "that is of general applicability" and is "not aimed individually at Saudi Arabia for a specific requirement." Asked to elaborate, he said, "As far as I'm concerned, why should Saudi Arabia be the sole country to have a condition imposed on it that, 'No, you can't put this plane on this base or on that base.' We are just as much threatened as anybody."

Secretary of Defense Harold Brown, in a letter to the Senate Foreign Relations Committee on Friday, said the Saudi government has assured the United States that the F15s will not be based at Tabuk near the Israeli border but at other bases closer to Saudi oil fields.

After questions were raised following Friday night's Saud interview, U.S. officials contacted the foreign minister in Los Angeles, where he was interviewed by Cronkite. The White House later announced that "the foreign minister states that Secretary Brown's letter accurately describes the position of the foreign minister and the royal Saudi Arabian government."

In a related matter, President Carter, in an interview released yesterday, said he would like to see an Israeli-Egyptian summit meeting at the White House. "I'd love to see that happen. I'd love for them to get together in a summit, yes," the president was quoted as saying. "But it doesn't matter where they meet. That decision is up to them. I've talked to both of them about getting together again."

He quickly added in an interview with Trude B. Feldman, a White House reporter who specializes in Midcast affairs, that he did not want to inspire false hopes by implying that Egyptian President Anwar Sadat and Israeli Prime Minister Manachem Begin would accept such an invitation.

"They are quite independent," he said. "And apparently each of them overestimates my influence on the other."

Carter also rejected allegations that his Middle East policies are influenced by a U.S. need for Arab oil.