King Hussein, despite recurring hints that he is ready to take Jordan into U.S.-sponsored Middle East peace talks, is having second thoughts because of American inability to obtain Israel's military withdrawal from Lebanon, according to officials and diplomatic sources here.

Well-placed Jordanians and diplomatic sources stressed that the king is likely to announce his intention to negotiate only if the United States first undergirds its Middle East credibility by persuading Israel to withdraw from Lebanon.

The monarch's hesitation has developed despite reportedly receiving two letters from President Reagan that are believed to offer U.S. guarantees on the future of East Jerusalem and on the freezing of Israeli settlements in the occupied territories during peace negotiations. Moreover, the United States apparently has made clear that sophisticated U.S. weapons systems "would be easier" to obtain once Jordan has entered the peace process.

In a series of interviews last week, Jordanian officials pressed their view that Israel was deliberately stalling on Lebanon to thwart negotiations on the future of the occupied West Bank and Gaza Strip, which are Prime Minister Menachem Begin's prime concerns.

This view from Amman contrasts with more positive signals in Washington and Cairo about Hussein's willingness to take a seat at the conference table. Those assessments appear to reflect more a decision in principle to bargain but give insufficient weight to his reluctance to give a public signal because of the continuing Lebanon impasse.

The fact that Hussein is leaning toward joining Reagan's peace initiative, at least in principle, and assuming a breakthough over Lebanon nevertheless is a clear contrast with his refusal in 1978 to join the U.S.-backed Camp David accords that eventually led to Egypt's separate peace with Israel.

His new thinking is said to reflect growing fears generated by the greatly accelerated pace of Israeli settlements in the occupied territories, the perceived threat of Israeli aggression against Jordan's East Bank and other threats to the kingdom that he has ruled for three decades.

The king, at one point, according to well-informed sources, was prepared to move decisively to outmaneuver Israel and Syria, both of whom rejected the Reagan initiative and had their own reasons to keep their armies in Lebanon.

By offering to negotiate and with the hope of much-needed support from the Palestine Liberation Organization and major moderate Arab countries, the king, according to Jordanian officials, wants to prove to the world that in the event of failure the blame should not be laid at the Arabs' door, but at those of the Americans and Israelis.

As Jordanian officials view events, past American reluctance or inability to hold Israel in check prompted the king's recent willingness, in a mood approaching last-minute desperation, to negotiate. But unless the United States moves promptly in Lebanon, they said, the king's position on peace negotiations and Reagan's Sept. 1 initiative for a Middle East settlement linking Jordan to the West Bank and Gaza risks being fatally undermined.

The apparent failure of special envoy Philip C. Habib's recent two-week effort in the Middle East to persuade Israel to withdraw from Lebanon has cast a pall on American credibility, which already has been damaged by the war in Lebanon and the increase in aid to Israel voted by Congress in December, Jordanian officials said.

Yet, as recently as Christmas, when Hussein returned from a visit to Washington where he conferred with Reagan, the monarch was exuding confidence about U.S. determination to back him. He had already brushed off warnings from new Soviet leader Yuri Andropov that Jordan risked undertaking burdens it could not bear by cooperating with the Reagan initiative.

Hussein was so convinced by Reagan's personal commitment to bring about the removal of all foreign troops from Lebanon within two months that he announced he would make known his negotiating intentions by March 1. A second trip to Washington was planned for that month.

The king's announcement of his deadline, made in a meeting with Jordanian local leaders Jan. 10, was described by diplomatic sources as "the final scene of the last act" in Hussein's plan to marshal every opportunity to force the PLO to join the negotiations on his terms and obtain active Arab support.

Although the king observed the 8-year-old Arab League custom of calling the PLO the "sole legitimate representative of the Palestinian people"--and added the designation "our partner and our colleague"--he made it clear in his speech that he felt PLO leader Yasser Arafat should drastically reduce his demands. The stakes were to "save the land" of the West Bank, which Israel "was in the last stages of swallowing," he said.

By Jordanian calculation, Israeli settlements now cover over half the West Bank--compared to about a third in 1978--and the number of Israeli settlers is expected to double in the next three months, then double again within a year.

What the king was doing, in still inconclusive negotiations, was in effect asking Arafat to step aside and abandon the dream of an independent Palestinian state, Jordanian officials said. Arafat was being asked, in the words of one official, to accept "self-mutilation" for the greater good of preventing an Israeli takeover of the occupied territories.

With a similar mission in mind, Hussein recently traveled to Iraq, Saudi Arabia, Bahrain, Qatar and Oman, trying to line up support for the Reagan proposal to associate Jordan with the West Bank and Gaza.

Despite official Jordanian optimism, some observers here noted a lack of enthusiasm, especially from the Saudis. As authors of the 1981 Fahd plan, which provided the main ingredients for the Arab League's reworked proposals at their summit in Fez, Morocco, in September, the Saudis apparently still favor a pan-Arab approach. Both the Fahd and Fez plans call for an independent Palestinian state, which the Reagan initiative ruled out.

Banking sources have attributed Saudi slowness in providing the Amman government with promised budget support in the past four or five months with their possible opposition to Hussein's approach. Jordan depends on Arab aid for more than half of its budget.

The United States, on the other hand, has employed a number of devices to press Hussein to join the negotiations, including the two letters from Reagan.

Their contents have not been made public, and Jordanian and American officials here even refused to acknowledge the existence of a second missive. They are believed to offer U.S. guarantees on the future of East Jerusalem--which is claimed by Arabs and Israelis--and on freezing Israeli settlements in the occupied territories during the peace talks. It was unclear how the United States proposed to make those guarantees.

An additional American incentive under discussion involves sophisticated U.S. weapons systems such as F16 aircraft and more Hawk antiaircraft missiles, which Israel has opposed providing to Jordan. In the past, Israeli pressures succeeded in restricting the normally mobile Hawk batteries to concrete silos.

Officials of both countries refused to discuss any new sales of sophisticated American weapons. But a well-placed Jordanian made clear that obtaining them "would be easier if we accepted the peace process."

Jordanian officials also are worried about the social, economic and political effects of a massive influx of Palestinians leaving the West Bank in the face of Israel's settlements policy.

It was this pessimistic evaluation that prompted the king to favor the peace negotiations, as much to save his East Bank as to preserve what could be preserved in the occupied territories.

"We have no choice," one official said, as he predicted the overthrow of moderate Arab governments throughout the Middle East unless the United States took forceful action soon.

From what can be pieced together, the United States is pledged to bring about a freeze on Israeli settlements in the occupied territories as soon as the king announces his willingness to enter the talks.

Jordanian officials concede privately that the king's March 1 deadline could slip by a month or so. The monarch set the original March 1 date because he felt that thereafter the United States would become so bogged down in presidential campaign politics that Israel would be able to freeze Middle East negotiations until after the 1984 voting.

The king hoped that Arafat would have his mandate renewed and thus be able to force through support for joining Jordan in negotiations at the meeting of the Palestine National Council, or parliament-in-exile, scheduled to begin Feb. 14 in Algiers.

A meeting of the PLO's executive committee in South Yemen this week apparently avoided a further split by coalescing support around the Arab peace proposal approved at Fez and rejecting any plan that does not recognize Palestinian rights to self-determination and a homeland. The meeting, however, did not reject outright the Reagan initiative. Long-time observers of PLO politics noted that this could continue to give Arafat some room for maneuver in contrast to an irrevocable split that might leave him powerless.

The exact state of ongoing Jordanian-PLO negotiations remains unclear despite optimistic, if vague, statements from both sides.

A member of the PLO executive committee said this week that the PLO still wants to be represented formally in any peace negotiations. The king is said to insist on public PLO endorsement of the joint negotiating team. But Arafat may prefer tacit approval to avoid admitting that the PLO has been excluded from the proceedings if he finally gives in to the king on eliminating formal PLO representatives.

The United States has made known it wants no PLO involvement. Jordanian officials said they found this stand unrealistic and dictated by fears of Israeli objections to the PLO.

If Arafat is unwilling or unable to reach agreement with Jordan, some Jordanian officials believe the king may try to form a negotiating team made up of West Bank residents, Gaza residents and Palestinians in the diaspora. To bear any chance of success, going it alone would require Arab world consensus and especially Saudi acquiescence, according to diplomats. The Saudi tie with the PLO is such that the king may realize that gambit is doomed, especially in light of his financial dependence on the Saudis.

Jordanian concern about Arab backing for even an agreement with the PLO was made clear by Crown Prince Hassan, who suggested in an interview that a special emergency Arab summit should be called to approve such a deal.