President Reagan has been assured by Jordan's King Hussein and Saudi Arabia's King Fahd that his Middle East initiative is still alive, U.S. officials said yesterday.

Reagan, who talked by telephone with Hussein, Fahd and Morocco's King Hassan, is trying to rally moderate Arab leaders to pressure the Palestine Liberation Organization into giving Hussein a mandate to enter peace talks with Israel, the officials said.

They described Reagan as determined to prevent what they called a "radical minority" within the PLO from vetoing an agreement that would permit Hussein to negotiate for the Palestinians on the future of the Israeli-occupied West Bank and Gaza Strip.

Administration spokesmen stuck to that position yesterday in the face of repeated questions from reporters about whether, in reality, the president's initiative has been effectively killed by the events of the weekend and whether the United States should now devise some new approach to the Arab-Israeli conflict.

"The president is encouraging all Middle East leaders to work together," White House spokesman Larry Speakes said. "He will not be deterred. He will leave no stone unturned."

State Department spokesman John Hughes added: "We are determined to go forward with the Sept. 1 initiative announced by Reagan last year . . . . As a result of those conversations with Hussein and Fahd , the president is confident that the process is going forward. He is confident that they have not given up."

The drumfire of upbeat rhetoric coming from U.S. officials outwardly seemed to contradict reports from the Middle East indicating that Hussein has washed his hands of efforts to draw him into an expanded peace process, unless Hussein's moves are seen as part of the effort to put pressure on the PLO. U.S. officials said publicly yesterday that they could not speak for Hussein.

Since the Reagan initiative calls for the occupied territories to be given independence "in association with Jordan," Hussein's participation is essential to progress on the U.S. proposals.

On Sunday, Hussein said he would not enter the peace talks proposed by Reagan last Sept. 1 because the PLO was seeking to change an agreement that he reached last week with PLO leader Yasser Arafat. In his statement Sunday and comments yesterday, the king indicated that he was pulling out of the process and leaving to the PLO the problem of trying to bring the occupied territories back under Arab control.

However, administration officials said yesterday, Reagan, who talked with Hussein twice on Sunday, is convinced that the king still wants to come in and has the backing of Arab moderates such as Fahd.

The officials added that the United States also agrees with Hussein that he needs a green light from the PLO before the Arab world will recognize him as a legitimate spokesman for the Palestinians.

As a result, the officials said, there is a need for the moderate Arab states supporting Hussein to put pressure on what the administration insists is "a radical minority" within the PLO to stop blocking an agreement between the king and Arafat.

The administration yesterday stuck to this argument in the face of skepticism from the press and others who pointed out that the opposition in the PLO to giving Hussein a mandate appears to involve more than a "minority" and includes Arafat's own Fatah faction, the biggest force within the PLO.

Nevertheless, said the officials, the stance now being taken by Hussein should put the PLO on notice that it has no chance on its own to fulfill its aspirations for a Palestinian homeland and that its only hope is to cooperate with Jordan.

The officials said it probably will require at least a few days to see whether Hussein and his moderate allies are getting that message across sufficiently to bring the PLO back to the original Hussein-Arafat agreement.

An Arab League summit meeting tentatively is planned for this weekend in the Moroccan city of Fez. The officials said that if the meeting does take place, it is likely to provide the first clear signs of whether the moderates have sufficient strength within the badly divided Arab world to force the PLO back on course toward an accommodation with Hussein.

The officials said that once there is a clearer picture of which side is stronger, the administration will decide what further moves to make.

Such ideas as sending Secretary of State George P. Shultz to the region are under consideration, but the officials stressed that it is too early to make any decisions about the next steps.

Shultz, the architect of the Reagan initiative, said yesterday in an interview with CBS News that, after Hussein reached an understanding with Arafat, the PLO, under pressure from its radical factions, came back with new demands for direct representation at the bargaining table and for prior commitment to making the occupied territories an independent Palestinian state.

These conditions run counter to the Reagan proposals. Shultz said they were unacceptable to Reagan, Hussein and Fahd; and he added: "Israel wouldn't sit down under those circumstances, and I don't blame them."

Shultz predicted that "in the end" a way will be found for Hussein to come into the peace process under conditions where "he is genuinely able to negotiate for the Palestinians on the West Bank and elsewhere."

Shultz' deputy, Kenneth W. Dam, speaking yesterday in St. Louis, warned that if the Arabs continue to delay, the expansion of Israeli settlements on the West Bank could have the effect of ceding the area to Israel without any negotiations.

"We will not be sidetracked by the events of the weekend," Dam said. "We will not permit radical elements to exercise a veto over the peace process. Rather, we will encourage the Palestinians to recognize that this is a unique moment that must be seized before it is lost."