President Carter welcomes the "unprecedented exchange" planned between the leaders of Egypt and Israel, which has come as an encouraging surprise, White House press secretary Jody Powell said yesterday.

American officials sought yesterday to wrap their own strategy around the dramatic leap toward a meeting between Egypt's President Anwar Sadat and Israeli Prime Minister Menahem Begin.

Diplomacy in the Middle East could be transformed by the Sadat-Begin development, many administration specialists said yesterday, even though a Sadat trip to Israel "obviously has high risks." The largest hazard is arousing exaggerated expectations that a Sadat visit could not fulfill.

The prevading view among U.S. officials is that the risks are outweighed by the sudden new prospect for piercing "the psychological barrier" of Arab-Israeli fear and suspicion.

State Department spokesman Hodding Carter III said the specific U.S. role in the Sadat-Begin exchange was to serve as "courier and conduit between the parties" to transmit the formal Israeli invitation to Cairo yesterday.

The actual message from Begin arrived there late last night, Cairo time. U.S. Ambassador Hermann Eilts was arranging a time to deliver it before Sadat's departure today for Damascus, and a crucial meeting with Syrian President Hafez Assad.

Israel's Ambassador to Washington, Simeha Dinitz, late yesterday delivered to Secretary of State Cyrus II Vance a letter from Begin for President Carter that included a copy of the Begin invitation to Sadat.

Dinitz told reporters the letter to Carter included "appreciation" for the U.S. role in transmitting the invitation to Sadat. Other sources said Begin's letter to the President also sought to assure coordination between American and Israeli policy, for what is developing now is not predictable clearly in anyone's diplomatic script.

The United States is intent on pursuing the existing pattern of American strategy toward "an overall settlement of the Middle East conflict through a Geneva conference," State Department spokesman Carter emphasized yesterday.

He said, "we welcome" a Sadat visit to Israel as "a concrete contribution by the leaders involved" toward that goal, which is "to reconvene that conference as soon as possible."

There are others in Israel, an Israeli opposition leader visiting Washington said yesterday, who maintain that the Sadat-Begin exchange goes in a different direction from a "comprehensive" Arab Israeli settlement, with Egypt, Syria, Jordan and Lebanon.

This demostrates, this Israeli source claimed, why the priority should be on an Israeli-Egyptian settlement, as advocated by the former Labor government of Prime Minister Yitzhak Rabin. Rabin is in Washington and met Monday with Secretary Vance.

Israeli Ambassador Dinitz last night, when asked if Israel is prepared to have "separate negotiations" with Egypt if Sadat is prepared to do so, replied:

"Israel has always said that it will negotiate for the purpose of signing peace treaties with each and everyone of the Arab countries neighboring Israel. The prime minister of Israel, may I remind you, today in Parliament also invited the leaders of Syria, Jordan and Lebanon to visit Jerusalem for the same purpose . . ." He said Israel will negotiate "with any one who is ready."

Senior U.S. officials yesterday said they regard it as most improbable that Sadat will agree to a separate peace settlement with Israel. They said they do not regard a Sadat visit to Israel as any departure from the commitments of all parties to a "comprehensive settlement."

Sadat "is not going to Israel to have a separate settlement." Ashraf Ghorbal, Egypt's ambassador to Washington said yesterday. "That is not in the cards." Ghorbal said at a meeting with reporters sponsored by Foreign Policy magazine.