Israeli Prime Minister Menachem Begin said today he is awaiting decisions to be made in Saudi Arabia on the Syrian missile crisis and that when the United States receives themm, diplomatic efforts to ease the confrontation will continue.

Begin said he probably will meet Monday with U.S. special envoy Philip C. Habib before returns to Damascus in the third week of his shuttle effort to avert another Middle East war.

Although Begin did not reveal the substance of the contacts with Saudi Arabia, his prediction that the Habib mission will continue was cast in more upbeat language than that used by his aides yesterday, when they were saying time was running out.

"I will know what happened in Riyadh. Immediately, Mr. Habib will get the news from the capital of Saudi Arabia, and we'll see," Begin told reporters after a meeting of the Israeli Cabinet.

A spokesman for the Israeli Army command, meanwhile, issued a background paper saying several hundred Libyan soldiers armed with antiaircraft guns, ground-to-ground missiles, rocket launchers and heavy artillery have moved into Lebanon via Damascus.

The spokesman also said that Libya increasingly is supplying Lebanese Moselm militia forces with heavy weaponry.

There was no independent confirmation of this account, which also said Libyan soldiers are commanding antiaircraft batteries including AA9 missiles supplied by Tripoli to the Palestine Liberation Organization and deployed in the Damour region south of Beirut.

The Army command, quoting The Associated Press, said a convoy moved into Lebanon from Syria on Thursday night, including 54 Syrian Army trucks and 18 long-range guns. The Israeli Army statement, in its assessment of the AP report, said the trucks apparently were carrying Libyan-supplied weapons to Moslem forces in Lebanon.

[The State Department in Washington, asked about the Israeli Army paper, said it cannot confirm that Libyans are stationed in Lebanon.]

Begin later before an audience in Tel Aviv that Soviet advisers also accompany Syrian troops inside Lebanon. He did not specify what role the Soviets allegedly play in Lebanon, but linked them to Syria's tank forces. There also was no independent confirmation of this allegation.

Begin's earlier remarks in Jerusalem appeared to refer, in part at least, to discussions on the missile crisis held over the weekend at the Arab League foreign ministers' conference in Tunis. Besides Lebanon and Syria, Saudi Arabia is the only other Arab League country Habib has visited in his shuttle and the Saudis have been active in the dipolmatic effort to ease the crisis.

Begin also disclosed today that as early as August 1978, he made a commitment to the northern Lebanese Christian forces that if were attacked by the Syrian Air Force, the Israeli Air Force would intervene. Begin said the commitment was reaffirmed last April 8, three weeks before Israeli jets shot down two Syrian helicopters that the Israelis said had been used in attacks on Christians in the Mount Sanin area east of Beirut.

Disclosure of the commitment in the Israeli press today touched off a political storm here, and Begin said that because of leaks about the pledge by the parliament's Foreign Affairs and Defense Committee, he will reconsider whether he should give classified information to that committee.

In the latest of a series of confusing contradictions of statements by Israeli officials, Begin termed as "absolutely untrue" assessments given to reporters by a number of his senior aides last night to the effect that time in running out, for the diplomatic effort, and that if Habib fails to achieve a breakthrough in his next visit to Damascus, Israel will be hard pressed to give the United States more time.

Top-ranking officials of the prime minister's office, in what appeared to be a coordinated effort to signal Israeli impatience with the slow pace of the negotiations, said the Syrians, the PLO and Libyan forces were "exploiting" the delays by redeploying armed forces throughout Lebanon. They called the redeployments "worrying" to Israel's security.

Despite the worrisome tone conveyed in the official leaks last night, Begin today said, "The reports are absolutely untrue. We don't lose patience whatsoever."

Begin's confirmation of a 1978 Israeli defense commitment to the Lebanese Christian forces came in a denial of Israeli press reports today that the commitment was made in writing to the Christian military leader, Beshir Gemayel.

Begin said an Israeli delegation and a delegation of Lebanese Christians met in August 1978, and that when the Christians said they could not defend themselves against Syrian air attacks, the Israelis assured them that Israeli would use its Air Force to intervene if the Syrian Air Force attacked.

Although Israeli support of the northern Lebanese Christians has been known for several years, it was the first time the prime minister had confirmed a formal agreement of promised intervention.