President Carter said yesterday "there is a chance" that the Palestine Liberation Organization will recognize Israel's right to exist, implying that the United States would expect Israel to reciprocate by agreeing to PLO participation at a Geneva conference on the Middle East.

In a nationally televised news conference, the President stressed the United States' "special relationship" with Israel and said there is still "a long way to go" before peace in the Middle East is achieved.

Earlier in the day, Carter gave concrete support to this "special relationship," agreeing at a meeting with congressional leaders to include Israel among those nations allowed to produce American-designed weapons from plans and technology supplied by the United States.

Just back from a five-day trip to Europe, which included a four-hour meeting with Syrian President Hafez Assad, the President said he felt encouraged about the prospects of removing one of the "major obstacles" to convening a Geneva conference on the Middle East later this year.

That obstacles is the PLO's refusal to recognize the right of Israel to exist. Israel, in turn, refuses to deal directly with the PLO, whose participation at a Geneva conference is considered crucial for meaningful negotiations.

Carter's guarded optimism on this point seemed all the more significant in light of recent reports - never denied - that the Soviet Union has informed the United States the PLO is ready to recognize Israel's right to exist in return for Israeli acceptance of an undefined "homeland" for the Palestinians in the Middle East.

Sketching the historical background of the complex situation, the President left the clear implication that the United States would expect Israel to reciprocate if the PLO made the first move.

"Our government, before I became President, promised the Israeli government that we would not recognize the PLO by direct conversations or negotiations as long as the PLO continued to espouse the commitment that Israel had to be destroyed," he said.

"I would like to see this resolved," he continued. "There is a chance that it will be done. We are trying to add our efforts to bring this about. But I have no assurance it will be accomplished."

In London, however, Israeli Foreign Minister Yigal Allon yesterday flatly ruled out acceptance of "a separate Palestinian state" as part of a settlement, saying Israel would accept only a Palestinian federation with Jordan.

During the press conference, Carter reiterated his support for providing a homeland for the Palestinians, calling it crucial to a peace settlement.

"I don't think there can be any reasonable hope for a settlement of the Middle Eastern question . . . without a homeland for the Palestinians,' he said. "The exact definition of what that homeland might be . . . (has) to be worked out by the parties involved. But for the Palestinians to have a homeland and for the refugeee question to be resolved is obviously of crucial significance."

Clearly hoping to calm Israeli nervousness over this issue, the President also pledged that the United States' "special relationship" with Israel "will be permanent as long as I am in office."

"It is absolutely crucial that no one in our country or around the world ever doubt that our No. 1 commitment in the Middle East is to protect the right of Israel to exist, to exist permanently and to exist in peace," he said.

White House officials originally had scheduled the press conference for last night, for prime time television, hoping to capitalize on Carter's successful trip to Europe. It was rescheduled to the afternoon so as not to compete with the David Frost interview of former President Nixon.

Carter said his meetings with European leaders had produced "a renewed spirit of hope and confidence" within the Western democracies.

The President's meeting yesterday oon the F-16 issue in Israel was sparked by a proposed amendment to the pending foreign aid bill by Sens. Hubert H. Humphrey (D-Minn.) and Clifford P. Case (R-N.J.). The amendment, designed to circumvent White House policy, would have affirmatively asserted Israel's right to "coproduce" the plane with the United States.

During the meeting with Humphrey and others, Carter agreed to add language to a White House memorandum on the subject assuring that "particular consideration . . . must be given to our military arms and coproduction arrangements with Israel."

Humphrey and Case agreed to withdraw their amendment and the Senate Foreign Relations Committee later in the day adopted a milder amendment on the subject.

With the Israel matter resolved, the committee went on to approve a $4.7 billion foreign aid bill. The only significant amendment at yesterday's sessions came when Sen. Dick Clark (D-Iowa) proposed an additional $52 million for Botswana, Lesotho, and Swaziland to aid those countries in dealing with refugees from South Africa.

Clark's amendment was unanimously adopted.