The Carter administration yesterday welcomed the decision of Egyptian President Anwar Sadat and Israeli Prime Minister Menahem Begin to concentrate on a Geneva conference format for a broad Arab-Israeli peace settlement.

Sadat and Begin both designated President Carter as the diplomatic middleman, to help carry forward the momentum of Sadat's psychological-breakthrough trip to Israel.

U.S. officials said yesteday that it is too early to say what changes will be made in the route to the elusive Geneva conference as a result of the Sadat mission. The hesitancy is not simply Reluctance to state a course publicly, officials said, but a need for "time to assess" the next step.

Begin telephoned the President about 3:15 p.m. yesterday for an initial briefing on the extraordinary events over the weekend, a White House spokesman said.

In a conversation that lasted 5 to 10 minutes and was described as "warm and encouraging," the spokesman said Begin thanked Carter again for his efforts in the Middle East which Begin said set the stage for Sadat's historical visit. Carter expressed his appreciation for the call, and Begin indicated he would send a more detailed report later on his talks with Sadat.

No call from Sadat was reported during the day.

Earlier, White House Deputy Press Secretary Rex Granum said "We feel everything the leaders did is conducive to a broad-based Middle East peace solution - they were talking actions that will lead us to Geneva."

"We feel," the spokesman continued, "their actions break down a psychological barrier to peace in the Middle East and clearly their actions change the approach to seeking a just and lasting peace."

Asked what was meant by a changed approach, Granum hesitated and then said, "the situation clearly has changed," but there is no change in the basic U.S. approach to a settlement.

Administration policymakers are trying to determine for themselves just what has changed. Spokesman at the White House and State Departments spoke of a period of reassessment.

Two kinds of reports are awaited. There will be reports from the American ambassadors in Israel and Egypt on the private talks between Sadat and Begin. This will determine if openings for diplomatic movement exist behind the still-stalemated public positions set out by the two leaders.

In addition, American strategists are especially anxious to learn the attitude of very rich and influential Saudi Arabia toward the Sadat mission, and also the post-visit attitude of Syria. Saudi Arabia, a powerful force in the Middle East and financial supporter of Egypt, originally expressed "surprise" about the Sadat trip. Syrian President Hafex Assad strongly opposed the trip, but Assad has not stated whether he is now more or less opposed to a Geneva conference.

Alfred L. Atherton Jr., assistant secretary of state for Near Eastern and Southern Asian affairs, said yesterday that it is "terribly important at this stage not to let the momentum (of the Sadat visit) that has been achieved be lost."