King Hussein of Jordan, using language that U.S. officials said could open the way for the United States to talk to the Palestine Liberation Organization, told President Reagan yesterday that the PLO is prepared to negotiate with Israel on terms that recognize its right to exist.

In an important concession to Hussein, administration officials indicated that the United States might end its objections to his call for new peace talks under the "umbrella" of an international conference including the Soviet Union. Until now, the U.S. position has been that Soviet participation would not be helpful.

Hussein said he had the approval of the PLO for his announcement, including an endorsement from the PLO's Central Council yesterday in Tunis.

He called for peace negotiations between Israel and a joint Jordanian-Palestinian delegation on the basis of U.N. Security Council Resolutions 242 and 338, which assure Israel's right to exist within secure boundaries. The PLO has previously balked at accepting the resolutions or making an explicit acceptance of Israel's right to exist.

It was not immediately clear whether Hussein's commitment on behalf of the PLO would lead to a breakthrough in the long-stymied effort to launch Arab-Israeli talks. The United States refuses to deal with the PLO until it accepts the U.N. resolutions, acknowledges Israel's right to exist and renounces the use of terrorism.

Reagan publicly reserved judgment on Hussein's proposal, which the king made as the two held an impromptu news conference on the White House south lawn.

"We have made it very plain that nothing has changed those conditions under which we would meet with the PLO," Reagan said. And a senior U.S. official, who spoke with reporters later, stressed that the PLO would have to affirm explicitly the commitment made by Hussein on its behalf.

But U.S. officials regarded Hussein's comments as a potentially significant development. The senior official told reporters, "I think you have heard a significant statement which was fully coordinated by the PLO leadership." Another U.S. official added, "What the king said moves the PLO pretty far along toward the conditions long set by the United States."

The officials cited as particularly encouraging the fact that Hussein had discussed the situation with PLO Chairman Yasser Arafat before coming here. Those discussions had led to the meeting of the PLO Central Council, which endorsed a joint peace effort with Jordan calling for the Israeli-occupied West Bank and Gaza Strip to become a Palestinian state in confederation with Jordan.

However, the first fragmentary reports from Tunis indicated that the council might have attached some potentially troublesome qualifications, including a demand that the confederation be preceded by creation of an independent Palestinian state -- a concept unacceptable to Israel and opposed by the United States. In addition, the PLO has a long history of making statements that seem to imply a recognition of Israel and then backing away from them.

The 71-member Central Council oversees policies of the PLO between the infrequent meetings of the Palestine National Council, which serves as the organization's parliament.

After the three-day meeting, which was boycotted by Syrian-backed PLO rebels, PLO spokesman Ahmed Abdul Rahman said the Central Council insisted that any Palestinians who meet with the United States in a joint delegation with Jordan "should be PLO representatives." He did not elaborate.

Until yesterday, U.S. soundings -- including a trip to the Middle East by Secretary of State George P. Shultz earlier this month -- had indicated that many obstacles remained in the way of naming a Jordanian-Palestinian delegation able to meet the American conditions and make possible later direct talks with Israel.

On Tuesday, the same senior official who yesterday characterized Hussein's remarks as "a significant statement," had cautioned reporters "not to look for dramatic occurrences in the next 48 hours."

U.S. officials, speaking on condition they not be identified, said that assessment began to change when Shultz talked with Hussein at length Tuesday night and was informed of the king's recent talks with Arafat. They added that Hussein's elaboration in the White House meetings yesterday, including his stress on the PLO council action in Tunis, had fueled optimism about a possible breakthrough.

In fact, the seemingly impromptu news conference following the Reagan-Hussein meeting was orchestrated by White House officials. They told reporters that the two leaders would be willing to answer questions after making their formal farewell remarks, and the subsequent questioning allowed Hussein to say that he and the PLO were ready to negotiate on the basis of "all pertinent U.N. resolutions including 242 and 338."

In addition to that point, U.S. officials also cited as encouraging several of his other remarks to reporters. According to the officials, these included:

* The king's endorsement of Reagan's statement that he hopes direct negotiations with Israel can begin "by the end of this year."

* Hussein's statement that he and the PLO have a "genuine desire for negotiations proceeding in a nonbelligerent manner." The Jordanian delegation later issued a clarification changing the word "manner" to "environment." But they and U.S. officials affirmed that the intent was to imply a willingness to accept Israel's existence in line with the Resolution 242 formula commonly known as "exchanging land for peace."

* Hussein's reaffirmation that land returned to the Arabs would become part of a Jordanian-Palestinian confederation rather than an independent Palestinian state.

* His use of the term "umbrella" in describing the international conference proposed by Jordan and his reference to the multination 1973 Geneva conference on Middle East peace as a model. The Geneva conference led indirectly to the successful Camp David accords between Israel and Egypt, and U.S. officials interpreted Hussein's reference to Geneva as an indication that he sees an international conference as a starting point that ultimately would turn into bilateral Arab-Israeli talks.

In response to questions about Hussein's call for an international conference that would include the five permanent members of the Security Council, Reagan said: "We have not resolved some of the differences. We are still discussing the whole matter."

"We're going to continue discussions about that question," the senior official said in his briefing. "We are looking for a process that has to be structured so that it contributes to the peace process -- one that doesn't detract from it, doesn't derail it."

That was an apparent reference to past U.S. and Israeli concerns that if the Soviet Union becomes a participant, it will seek to enhance its influence among hard-line Arab states by obstructing progress toward peace.

However, another U.S. official acknowledged that "the United States obviously has changed its position about an international conference out of appreciation for Hussein's efforts and recognition of his need for some sort of international blessing for his taking on the politically risky task of direct negotiations with Israel.