President Carter yesterday moved to expand his successful peace initiative in the Middle East issuing a preliminary call for an international conference to resolve the 3-year-old civil war in Lebanon.

Clearly buoyed by his success in bringing Egypt and Israel to peace terms at Camp David, a relaxed and confident-sounding Carter told a news conference that Lebanon's warring factions and interested countries should now take joint action that could lead eventually to a new Lebanese constitution and a strong central government.

Describing separate telephone conversations he had yesterday with Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin, Carter also said that the two leaders assured him that there are now "no remaining obstacles" to the signing of full peace treaty before Dec. 17.

The president left open the possibility that he would travel to the Middle East for the treaty-signing ceremony, and said he is sending a high-level U. S. delegation to the first formal Egyptian-Israeli treaty talks, due to open within two weeks.

Carter underscored his optimistic view of the complex process outlined in the two framework agreements signed at Camp David by saying, in response to questions, that a continuing dispute between him and Begin over future Israeli settlements on the West Bank "will certainly be no obstacle to the progress toward peace."

The Israeli leader sent Carter a message yesterday reiterating that he has not agreed to an arrangement that would have given Arab negotiators an effective veto over new Israeli settlements for up to five years.

Carter called the distance between this view and his assertions that Begin did agree orally at Camp David to such an arrangement "an honest difference of opinion."

Movement toward the treaty also came from a decision by Carter to send Israel a letter committing the United States to pay for and help in the construction, if necessary, of two military air bases in the Neger Desert to replace three bases the Israelis are being forced to abandon by returning the Sinai peninsula to Egypt.

The letter, signed by the Secretary of Defense Harold Brown, was sent yesterday, after being held up earlier this week as the dispute over the settlements on the West Bank sharpened.

The president's calls to Begin and Sadat followed the Israeli Knesset's (parliament) vote early yesterday to approve the two framework agreements and the dismantling of Israeli settlements in the Sinai, which Sadat had described as prerequisite for the peace treaty. Carter repeated praise for the Knesset's "courageous action."

The treaty is to be signed within three months of the Sept. 17 ceremony that ended the 13-day Camp David summit. The signing of the treaty triggers a partial pullback of Israeli forces in the Sinai, the establishing of normal diplomatic relations between Egypt and Israel and a full return of the Sinai to Egypt within three years.

The United States will continue to be "a full partner" in the talks on the peace treaty, Carter said. Although he does not expect to be personally involved in the negotiations leading up to treaty unless a major dispute develops between Sadat and Begin, the president added that "Nothing would please me more than to participate in the signing of a peace treaty at an early date." He noted that he had promised Sadat to visit Egypt at some future time.

The president's initiative on Lebanon, which White House aides emphasized last night is still in a very tentative phase, would have the effect, if successful, of providing additional protection for the Camp David agreements by bringing a cease-fire to a war that has involved almost all of the major actors in the Arab-Israeli conflict.

Carter disclosed that he, Sadat and Begin discussed the Lebanese war, which is estimated to have cost at least 100,000 lives, or more than the four Arab-Israeli wars combined. On their helicopter ride down from Camp David to sign the peace agreements Sept. 17.

"All three of us committed ourselves to renew our support for the government" of President Elias Sarkis, Carter said. The Sarkis government is a compromise cabinet that is caught in the continuing cross-fire of Christian militiamen supported in part by the Israeli army. Palestinian guerrilla forces and Moslem Lebanese.

The Syrian army provides the bulk of troops for an Arab peacekeeping force that has increasingly become involved in the fighting.

Diplomatic sources said the Carter administration has been sounding out Arab and European governments since the conclusion of the Camp David summit on ideas for getting a cease-fire in the war.

These sources said that the concept of a conference had not been agreed upon in the preliminary soundings, which are continuing at the United Nations, where Secretary of State Cyrus R. Vance is due to deliver a major address today.

After listing the Arab countries involved, Israel, the United States and France, the latter the former colonial power in Lebanon, Carter said:

"I think it is time for us to take joint action to call a conference of those who are involved, primarily the people who live in Lebanon, the different factions there and try to reach some solution that may involve a new charter for Lebanon." This could be done, Carter suggested, under the United Nations aegis.

The basic document of government for Lebanon, which is more of a power-sharing arrangement between the country's nearly equal Christian and Moslem populations is called in Arabic the National Charter.

It was signed in 1943, and its failure to resolve power-sharing disputes in the government was a prime cause of the civil war that has destroyed Beirut, once the Middle East's most Westernized city.

The Lebanese ambassador in Washington, Najati Kabbani, said he had not been consulted by the United States on an international conference, but he praised Carter's statement, "Any initiative to bring peace to Lebanon is welcome," he said.

The first question Carter faced in his 37th news conference as president was about Begin's continuing insistence that he had not agreed, as Carter has publicly said he did, to an arrangement that would require agreement from Egyptian, Jordanian and West Bank Palestinian negotiators before new Israeli settlements can be established on the West Bank.

"I can only use persuasion and depend upon the mutual trust that exists between me and him," Carter said. He quoted a statement made by Israeli Foreign Minister Moshe Dayan upon his return to Israel that referred to a five-year period in which the settlements "will come up and will be discussed and agreement will have to be reached on this subject."

The president said he had received a new letter from Begin, which failed to resolve the dispute, but aides said later that Begin had only sent an oral message to Carter through the U.S. embassy in Tel Aviv.

U.S. and Israel officials confirmed that Begin is sticking by his interpretation that the moratorium on settlements he promised is limited to the three-month period it will take to negotiate the Israeli-Egyptian agreement.

Testifying on Capitol Hill yesterday, Assistant Secretary of State Harold H. Saunders, the State Department's top Middle East expert, said that, for serious negotiations on the West Bank and Gaza Strip to go forward, Israel would effectively have to accept a moratorium on settlements for the period of those negotiations, which could go on for five years.

"The understanding we have is that as long as serious negotiations are going on there would be no new settlements." Saunders told a House International Relations subcommittee.

In response to a question. Saunders said that he understood that Vance had been assured by Saudi Arabia and Jordan last week that those two countries would do nothing to obstruct the Camp David agreements or to undermine Sadat.