A new Saudi Arabian request to improve the combat capability of its U.S.-supplied F15 fighter planes generated strong opposition yesterday from Israel and its supporters in Congress, posing painful choices for President Carter.

A replay of the 1978 Saudi-versus-Israel struggle over the F15s would pit the pride, prestige and power of America's top overseas oil supplier against that of America's most politically potent ally in the Middle East. That is just the kind of conflict that a U.S. administration would like to avoid in an election year, but such a new test is shaping up nonetheless, officials conceded.

White House and State Department spokesmen, in coordinated statements, confirmed that the Saudis have asked to purchase additional equipment for the warplanes, which the United States agreed to sell them two years ago.

As part of the effort to mollify congressional opponents of the sale, the administration agreed in 1978 not to supply supplementary equipment to increase the planes' range or ground attack capability. It is just such extra gear -- including additional fuel tanks, bomb racks and missiles -- that the Saudis reportedly requested in recent weeks.

Washington officials professed to be uncertain and somewhat puzzled about why the Saudis chose to press the issue at this time, and to make of it a major test of the value that the United States places on relations with the oil-rich kingdom.

Some Middle East sources are reported to have made the point that Israel obtains what it wants from the U.S. government in election years -- and that the Saudis have decided to do likewise.

News reports of the Saudi request stirred a quick response from opponents. Israeli Ambassador Ephraim Evron telephoned the State Department to protest any inclination to grant the sale of the additional items, saying that Israel would be the only target of the advanced weaponry, according to the Israel Embassy.

Chairman Frank Church (D-Idaho) of the Senate Foreign Relations Committee and Sen. Richard Stone (D-Fla.), chairman of the Middle East subcommittee, objected to the sale of the supplementary equipment.

Church charged that such a sale would violate the administration promise of 1978. Stone said administration assurances of 1978 would "make it very difficult" to sell the supplementary gear. Both men are strong backers of Israel and opposed the sale of the warplanes in the first place.

Administration spokesmen said it has not been decided whether to grant the Saudi request for the supplementary equipment. Saudi Arabia has yet to receive the first of the 60 F15s sold under the 1978 agreement because none has been completed.

Defense Secretary Harold Brown is scheduled to meet the Saudi defense minister. Prince Sultan, in Geneva June 26 and may be pressed then for an answer to the question. Though some U.S. officials would like to postpone a decision until after the November election, others expressed doubt that the Saudis will agree easily to such a delay.

State Department spokesman Thomas Reston, reading a prepared statement, suggested that "the increased threat posed by the Soviet invasion of Afghanistan" is a factor in the U.S. deliberations on the issue. A justification of greater danger in the region appeared to be the most likely rationale should the administration decide to approve the Saudi request despite the earlier assurances and the strong political opposition.

A renewed warplane procurement controversy would add to U.S.-Saudi tensions at a time of growing strain ove r the issue of filling the U.S. petroleum stockpile. A House-Senate conference committee on the synthetic fuels bill has decided to order President Carter to resume filling the oil reserve by Oct. 1, despite vocal Saudi opposition.

White House press secretary Jody Powell said Carter prefers "greater flexibility" on the date for resuming oil purchases for the reserve, but that "we think [the requirement] is something we can live with."

Despite remarks by some Saudi officials suggesting otherwise, an Energy Department spokesman said Saudi Arabia is unlikely to cut oil production if the United States resumes the purchase of petroleum to fill the reserve. The reserve now hold about 92 million of their proposed level of 750 million barrels of oil.