U.S. Officials cautioned yesterday that Egyptian President Anwar Sadat's visit to the United States beginning Friday is not likely to produce "major breakthroughs" in the search for peace in the Middle East.

Confirming the visit - which will begin with a weekend of private talks with President Cartet at Camp David - these officials insisted that the invitation to Sadat was entirely an American initiative. Sadat has been saying for several weeks that he would like to come here.

A senior official said President Carter feels he must be thoroughly expesed to Sadat's latest thinking if he is to continue trying to mediate between Egypt and Israel.

The trip is an important opportunity for Sadat in three ways, according to diplomatic sources here and in the Middle East.

It will allow the Egyptian leader to pursue the public relations campaign that is central to his "peace initiative."

It will give him an opportunity to urge Carter to pressure Israel to be more flexible in the stalled peace talks:

And Sadat will have a chance to press his request for American arms or other tokens of U.S. support for Egypt and - by implication - for him personally.

Jody Powell, Carter's press secretary and a senior official briefing reporters at the State Department both sought to portray the visit as just another in the series of face-to-face encounters between senior American officials and the leaders of the Middle Eastern countries.

Powell and the State Department spokesman both used the term "quiet diplomacy" to describe the Sadat visit. Both cited the use of Camp David for the talks as evidence of the American desire to keep them low-key.

But Sadat seems unlikely to use the "quiet diplomacy" terms. The Los Angeles Times reported from Cairo that a senior official there as saying: "The focus of Egyptian foreign policy is to increase pressures for settlement on Israel, and the major weapon of that policy will be President Sadat's public relations offensive in the American media."

Sadat is not limiting his target area to the United States. In conjunction with his trip here, Sadat will also visit Morocco, Britain, France, West Germany and Austria. Thomas W. Lippman of The Washington Post reported from Cairo last night.

An official in Cairo said Sadat hopes to elicit much stronger Western support for his position that it is now up to Israel to make concessions for peace.

But the main target, the same official said, was the American public, including the American Jewish community. Sadat is said to hope that he can renew the popular support he evoked by his dramatic trip to Jerusalem, and which he fears has wanted since, according to the Los Angeles Times report from Cairo.

U.S. officials said Sadat's schedule after the weekend at Camp David is still not set, so it isn't known what forums he might use to address the public here. But Sadat has shown a fertile imagination in the past on matters of public relations.

A senior American official said yesterday that Sadat would naturally try to take advantage of his presence here to make publicity, but added that the administration had no intention of helping him in that way.

The real purpose of the visit, this well-placed source said, was to give Carter and his associates an opportunity to try to see what is really on Sadat's mind now. The official noted that Sadat personally is really the only important figure on the Egyptian side, but that his temperament and unpredictability make it difficult to keep up with his latest ideas.

The Carter administration now feels it understands what Israeli Prime Minister Menachem Begin and his colleagues are thinking about the peace talks, but they aren't sure they understand Sadat - and particularly, why he broke off the political talks in Jerusalem.

"He's not as systematic in laying out his views" as Begin, this official said, yet he is more important personally to the search for peace.

In a report from Jerusalem, Ronald Koven of The Washington Post quoted an authoriative diplomatic source with fresh information on Sadat's views as saying the Egyptian will arrive in Washington in a mood of extreme irritation and impatience.

According to this source, Sadat will urge Carter to move beyond the mediator's role and press Israel directly to make new concessions. Sadat will say that time is running out on his peace initiative, according to this source.

The source said Sadat feels strong moral outrage at Israel's continued insistence on maintaining control over settlements and three airfields in the Sinai, and will speak especially bitterly on this issue to Carter.

While pressing Carter to in turn pressure the Israelis, Sadat is also expected to be asking for gestures of personal support for himself from the United States.

Sadat raised the question of American armaments for Egypts last Saturday, and he is expected to pursue it here. Specially, Sadat hopes the U.S. will provide him F-5 jet fighters.

Powell gave some encouragement to Egypt on this point at the White House yesterday. He said one of America's concerns in the Middle East was to provide for the security of this country's friends. Asked if Egypt should be considered a friend in that context, Powell said yes.

But Powell added that other considerations entered U.S. thinking on arms sales, and he made no promises of arms aid for Egypt.

Sadat is presumed to want some signal from the U.S. that the Carter administration appreciates and intends to reward his dramatic peace initiative. This reward could come in the form of pressure on Israel, arms aid or other economic assistance, or a combination of them all.

American officials' warnings not to expect great things from the Sadat visit were echoed in Cairo, where the foreign minister disputed reports that Egypt and Israel are moving closer to a declaration of principles to govern their peace talks.

Mohamed Kamel, the Egyptian foreign minister, told reporters in Cairo yesterday that there is still "a big gap" between Egypt and Israel. He said peace talks could last for months.

An American source indirectly supported this view, noting that although progress has been made on most of the six or seven "principes" the parties have been negotiating, they remain far apart indeed on the crucial one - the question of Palestinian self-determination.

The American assistant secretary of state, Alfred Atherton Jr., who is shuttling around the Middle East at the moment, is due in Cairo Monday to explain Israel's latest thinking on the statement of principles. Yesterday Atherton talked to King Hussein in Jordan.

Atherton will return to Washington for the Sadat visit.

The idea to invite Sadat to Washington at this time grew out of a meeting last Monday night involving Carter, Vance and other officials. It was the Secretary of State's first chance to report to Carter after returning from his last Mideast trip.