President Reagan's decision to summon home his special Mideast envoy, Philip C. Habib, does not reflect either confidence or despair about averting a new Isaeli-Syrian military clash but is a needed pause during a drawn-out negotiating effort, senior U.S. officials said yesterday.

From Reagan on down, officials went out of their way to say that Habib's mission has neither failed nor run its course, although at the same time there was no claim that the emissary's three-week effort is close to success. One senior official described it as still "a long shot."

A U.S. admission of failure would intensify political pressure on Israeli Prime Minister Menachem Begin to order military action against the antiaircraft missiles that Syria has placed in Lebanon's strategic Bekaa Valley.

One of Habib's most important accomplishments, according to a briefing for reporters at the State Department, has been to buy time by giving political leaders on all sides a rationale for delaying dangerous military steps.

A high-ranking State Department official said the United States has received no assurances that Israel will not attack the missile sites during Habib's withdrawal from the area for consultations.

Washington will not give either "a red light or a green light" to Israel for such military action, the official said. But he added that "Begin has increasingly become aware of the desirability of not having a conflict at this particular juncture."

The White House summons for Habib, who is expected in Washington tonight, was described by the high-ranking State Department official as "a return of convenience in many respects because there is a national break in the [mediation] process."

One reason for the pause, it was clear, was to assess the chances of an effort sponsored by Saudi Arabia to broaden mediation beyond the Bekaa Valley to encompass underlying problems and antagonisms of all major groups in Lebanon.

With this aim, the Saudis yesterday sent a resident ambassador Gen. Ali Shaer, back to Beirut for the first time in two years, smoothing his way with a special fund of 100 million Lebanese pounds (about $25 million) in aid for various Lebanese groups and factions.

The United States has not yet embraced the plan for broadening the basis of the mediation, which probably would involve extremely lengthy negotiations among the Lebanese groups and their regional sponsors. Nor has Israel agreed to it, according to diplomatic sources.

Israeli acceptance of such a long-range strategy for resolving the antiaircraft "missile crisis" likely would postpone a day of reckoning beyond the June 30 Israeli elections that will determine Begin's political future.

While Washington appears to be placing more hope in Saudi intercession than in direct agreement between Habib and Syrian President Hafez Assad, the Reagan administration appears anxious to preserve what is left of its dwindling tie with Damascus.

The Reagan budget for fiscal 1982, like President Carter's version of the same budget, did not request new aid funds for Syria, which became a U.S. aid recipient with the onset of U.S.-sponsored peace efforts in the mid-1970s. Secretary of State Alexander M. Haig Jr., in his Middle East trip last month, avoided the customary stop in Damascus.

In the midst of the present crisis, nevertheless, the State Department argued strongly last week against a move by Sen. William Proxmire (D-Wis.) to cut $25 million from the "pipeline" of previously approved aid to Syria.

Congressional sources said that Proxmire agreed to withhold his cut for the time being after State Department officials pleaded it would complicate Habib's mission and that Reagan and Vice President Bush were prepared to lobby against it for this reason.

The Soviet Union has increasingly become Syria's great-power sponsor, especially since the signing of a Syrian-Soviet peace and friendship treaty last October. In the present crisis, the Reagan administration has called on Moscow, like virtually all other parties with an influence, to exercise and counsel restraint.

At the State Department briefing, an official told reporters that Moscow had been "basically, not particularly helpful" so far in resolving the crisis.

Begin charged Sunday, and the Soviets promptly denied, that Soviet military advisers are with Syrian forces in Lebanon. The United States has received unconfirmed reports, according to a senior State Department official, that some Soviet technicians were sent into Lebanon for daytime stays to remedy technical breakdowns in the Soviet-supplied SAM6 antiaircraft missiles but were withdrawn before nightfall.