Days ago, back in Washington, Prime Minister Menachem Begin presented President Carter with a toughened stance on Sinai oil fields, a signal that the collapse of Iran and regional security concerns are playing a major role in the fragile Middle East peace effort.

Administration sources here with Carter say Begin wanted Egypt to guarantee that it would sell all the oil the Sinai fields now produce, plus all the oil the wells would be capable of producing in the future.

These Israeli demands first were made last fall, soon after the original Camp David accords. During subsequent negotiations, howerver, it was agreed that they sould not stand in the way of a peace treaty, and Israel did not push them.

But the downfall of the shah in Iran, the warfare between the Yemens North and South and the sensitive position of Saudi Arabia in the events all around its borders have pushed Israel and Egypt both to press more firmly their demands in matters of regional security, the officials say.

So back in Washington, Begin pointedly told Carter that the situation concerning oil had changed, according to one administration official. And in that Begin was correct, he said.

What had changed is that Iran, which formerly provided Israel with about 60 percent of its oil, has decided to stop all oil sales to Israel. Because of that, Begin said he wanted to modify Israel's position on the Sinai oil fields that it now controls but that would be turned over to Egypt in a peace treaty.

At Camp David, Egyptian President Anwar Sadat had promised only that Egypt would negotiate commercial oil sales fairly with Israel once it regains control of the wells, treating Israel the same as all other international clients.

American officials point to Israel's renewed emphasis on the question of the Sinai oil wells -- not because they believe it will prove an insurmountable stumbling block to an Egyptian-Israeli peace treaty but because it illustrates how the turnoil in the Persian Gulf region has added one new layer of difficulties to the talks in this latest stage of the Carter peace mission.

Iran has added to our concerns," said a Carter administration official. "And so have the problems in Yemen. They ar in themselves a major irritant to the other nations in the region. It is our view that the accomplishing of a peace treaty here would be less of an irritant to the other nations -- say to Saudi Arabia. Because it would lend a degree of stability. It would do away with at least one part of the to-ing and fro-ing in the region."

Sadat has been increasingly concerned about strengthening the security of not just Egypt but the entire region. So it was that Carter deliberately chose his national security adviser, Zbigniew Brzezinski, to brief Sadat on the latest U.S. proposals, rather than Secretary of State Cyrus Vance, who has handled most of the Middle East negotiations, according to one White House official

Sadat's concerns at this point are heavily along the lines of regional security, a Carter administration official said. And Brzezinski, who has publicly espoused a hard line against Soviet interference in the region, was said to have been sent to emphasize U. S. intentions to take strong action to protect U. S. interests in the area as well as the interests of American allies.

Brzezinski was not there to negotiate the details of the U. S. proposals in advance of Carter's visit, the official said. He added; "The president has begun to do something that he hadn't done before. That is to use -- and make use of -- the public perceptions of the distinction in styles of Cy and Zbig."

The United States is considering a major request for arms from Egypt. In other demonstrations of strong action in the region, the United States has announced that it is sending the U. S. S. Constellation aircraft carrier into the Persian Gulf; that it is speeding delivery of $100 million in tanks, aircraft and other weapons to North Yemen, which is engaged in a border conflict with Marxist Aouth Yemen; proposed to send a squadron of armed U. S. Air Force jets to Saudi Arabia in these times of tension in what would be officially described as a visit; and has agreed to supply combat jets to Sudan.

Also, Carter brought Defense Secretary Harold Brown with him to this diplomatic summit.

Sadat would like to see a sort of Marshall Plan for Egypt -- a Carter plan -- which would require $10 billion in American economic aid over a five- year period. The lesson of what happened in Iran was watched with great concern in many countrides in the region, Sadat's included. And today, as Carter's motorcade moved through central Cairo, the president saw hundreds of signs that read; "Stability and Prosperity."

Carter officials hope now, as they seek Sadat's agreement on the Carter proposals that have been endorsed by the Begin government, that the pressures for "stability and peace" may prove decisive in winning over Sadat. In Israel yesterday, Foreign Minister Moshe Dayan told a group of foreign ambassadors that Israel considers two items unsettled: The question of Sinai oil and an exchange of ambassadors with Egypt.

"If Egypt agrees to what Israel has already agreed to," Dayan was quoted as saying, "and if we can accept the ideas President Carter brings with him from Egypt on the points still not settled -- the exchange of ambassadors and the oil question -- we will be near a solution."

Meanwhile, information on the progress of Carter's effort at sudden summitry proved scarce. There was, for example, only one word culled on the record from any of the participants as to the nature of the subject at hand This was when Brzezinski and Defense Secretary Brown emerged from talks. Brzezinski looked very tired as he was talking with Brown, and a photographer, moving in for one last picture, heard the presidential adviser say one word: "oil" -- which he duly reported back to a waiting cluster of reporters.