Saudi Arabia's ambassador to the United States said yesteday he was "astonished at the implications" of a letter signed by 68 U.S. senators urging President Carter to reject the sale of extra equipment to Saudi Arabia for 60 American-built F15 jet fighters sold to them in 1978.

The statement by Ambassador Sheik Faisal Alhegelan made it clear that his government resents the lack of trust suggested by the letter. He warned that narrow U.S. concerns about Israeli security were jeopardizing Saudi Arabia's ability to defend itself and oilfields vital to western economies against threats from "all directions." a

He pointed out that since the original sale, events in the Persian Gulf-Southern Asia region -- including the Soviet invasion of Afghanistan -- had substantially increased the need for both defense and stability, and he implied that it is countries other than Israel that the Saudis are worried about.

The Saudis want to buy additional fuel tanks, bomb racks and aerial refueling equipment for the F15s, both to increase their round-trip range from about 450 miles to more than 1,000 miles and to add to their firepower.

The Saudis argue that, having paid $2.5 billion for the planes, they are entitled to have their full capability, especially since their country alone spans 2,000 miles and because of the new and unsettled conditions in the gulf. "For protection of such valuable real estate, you use your best weapons," a Saudi official said.

It was specifically those kinds of improvements, however, that were ruled out by the White House in 1978 after Congress made it clear that it would not agree to the F15 sale unless such assurances were given. Supporters of Israel in Congress, who feared that the extra range would constitute a threat to the Jewish state, insisted upon and got those assurances in writing.

In recent days, however, after the ne w Saudi request was revealed, the bipartisan group of senators wrote to Carter urging that he reject that request and reminding him of the previous assurances.

In his statement, the ambassador said "certainly relations between longtime friends sharing the most vital interests, as do the United States and Saudi Arabia, must be built on mutual respect, trust and a cooperative concern" for working out problems "on a just and responsible basis."

Saudi Arabia, he said, "with responsibility for the most holy cities of Islam, its own people and society, . . . and a fourth of the planet's proven petroleum reserves" must be able "to determine its own most basic policies to assure its defense."

More specifically, he said, "the kingdom must provide for its protection in all directions, on a 360-degree perimeter." To deny that in one "narrow-spectrum direction," meaning Israel, he said, "would limit Saudi Arabia's ability to defend itself in all directions."

Alhegelan said the "unavoidable implications" of the senators' attempt to limit Saudi defense "are further confirmation of the dangerous and destabilizing risks caused for the religion as a whole, America's longtime friends there, the international community and clearly the United States itself of allowing Israeli intransigence and continuing aggression to drag out and preclude a just and comprehensive settlement of the Arab-Israeli conflict."

The ambassador said: "It is long past time that the Israeli problem is placed in perspective with the profoundly more important defense, economic and other vital interests of the United States, its key friends, the international economy, world peace and certainly the overwhelming majority of people in the Middle East . . . ."

The ambassador also said that equipment is available from other sources without restriction or conditions, suggesting that Saudi Arabia might look elsewhere for its front-line armaments. The Saudis privately have implied that France may be interested in future weapons projects.

In the Saudi capital of Riyadh, the Saudi defense minister, Prince Sultan Ibn Abdulaziz, also delivered harsh criticism of the senators' letter.

Our reply to this," he said, accordint to Reuter news agency, "is that the kingdom will never ask for a weapon and be refused," implying that the Saudis would not take no for an answer and weren't going to withdraw their request.

At the State Department, spokesmen tended to play down the episode and offered no public comment on the Saudi statements.

'We read a lot of speeches. We see a lot of statements. We respond to some but not to others," department spokesman John Trattner said when asked about the Saudi statements.