Jordan's King Hussein today insisted that his floundering joint peace initiative with the Palestine Liberation Organization is still alive, but indicated that it is up to the United States to bring it to fruition.

In an interview, the king made it clear that he will honor his commitment to the PLO and reject any U.S. pressure to force PLO Chairman Yasser Arafat to water down his organization's role in proposed exploratory talks with Washington.

Relaxed but firm, the king said he would closely question Secretary of State George P. Shultz -- due here in mid-May -- about what is perceived here as a major new pro-Israel departure in U.S. Middle East policy.

Despite Assistant Secretary of State Richard Murphy's failure to meet with a joint Jordanian-Palestinian delegation here, the king insisted that "nothing is final" and "the dialogue is ongoing."

But Hussein made it clear that it is up to the Reagan administration to come up with new ideas for advancing the process.

"American credibility in the region is nearing its final test," he said.

Asked if the United States was sufficiently active in pursuing the opportunity offered by this initiative, the king said, "I do not believe the kind of effort we are now seeking from Washington is now apparent, but there is interest."

The king expressed the hope that Shultz would bring fresh U.S. proposals next month and that final details could be thrashed out during the king's visit to the United States in early June, when he said he hopes to meet President Reagan.

The current initiative stems from the Feb. 11 agreement between Jordan and Arafat's wing of the PLO to work together toward peace with Israel in exchange for a return of Israeli-occupied Arab lands.

Rather reluctantly, the Reagan administration was dragged into the new initiative at the behest of Egypt as well as Jordan. It has not, apparently, convinced Hussein as yet of its wholehearted involvement.

One major stumbling block is U.S. insistence that the PLO should not be represented directly in any joint delegation that would talk with Washington, although some Palestinians chosen with PLO approval, apparently would be tolerated.

The king said the Reagan administration "still is looking to find Palestinians falling out of trees who have no connection with the PLO."

He insisted that only the PLO can decide who represents the PLO, but also held out the hope that by the time he visits the United States there may be a list of acceptable names to present.

Arafat and the PLO executive committee as well as the central committee of the mainstream Fatah organization met in Baghdad, Iraq, 10 days ago, but failed to resolve the representation question.

The other major obstacle, the king recalled, is U.S. insistence that the PLO must specifically accept U.N. Security Council Resolution 242 which embodies the concept of land for peace and recognizes the right of all nations in the area to live in peace within recognized boundaries. So far, the PLO has refused to do so on the grounds that 242 treats the Palestinian people merely as refugees.

The Feb. 11 accord recognizes all relevant U.N. resolutions -- both in the General Assembly and Security Council -- as a bloc. The king said that it is understood these include 242.

"I believe American demands for clarification by the PLO of this are met," said the king. The Reagan administration says the PLO has not explicitly accepted the terms of the resolution.

He said, "No change is possible" in the Feb. 11 accord. "I am not accepting any change from the PLO and I am not accepting any change from anyone else. The accord is final."

The king said this was the crux of his two meetings with Murphy during the recently concluded two-week tour of the region by the assistant secretary of state for Near East and Asian Affairs.

"I cannot continue to be the person in between" the United States and the PLO, he said.

"These people," he said, speaking of the PLO, are "partners," stressing that he felt they had gone as far as they could.

"If the United States was under any illusion that ours was a bargaining position that would evolve or change," he said, that view should now have been dispelled.

"We are not playing a game here, we are serious," he said. "And we are working with the PLO."

Reiterating his more than decade-long formal commitment recognizing the PLO as the sole legitimate representative of the Palestinian people, he said the PLO was "in the process of transformation" and was now serious about "representing its constituency in the occupied territories and elsewhere."

That was taken as meaning the Arafat wing of the PLO had now abandoned armed struggle as policy.

The monarch, who spoke volubly in the interview in his downtown Basman Palace, noticeably weighed his words when asked about a recent Shultz speech to the Arab-Israel Public Affairs committee.

When an adviser noted that Shultz appeared to go beyond an 18-year-old U.N. resolution ruling out all but minor border changes, the king said, "this is not what they told us in 1967 when we were formulating [Resolution] 242 together."

"Arab territory is Arab territory," Hussein said, "and has to be returned to Arab sovereignty."

The adviser later indicated that the king was intent on clearing the air. He noted that the secretary had paid little heed to Jordan's repeated warnings that the secretary was risking disaster in the eventually ill-fated American-backed May 1983 Israeli-Lebanese withdrawal agreement.

Asked if he were now more optimistic, the king said, "Things looked impossible a few months ago, but hope has revived" thanks to the Feb. 11 accord. But he warned that "this is the last chance."