U.S. Middle East envoy Philip Habib conferred with Syrian President Hafez Assad on the Lebanese missile crisis, but tough statements in Jerusalem and Damascus indicated that he has failed so far to produce any visible change of Syrian or Israeli positions likely to avert the threat of hostilities.

The Syrian defense minister, Mustafa Tlas, told a special Cabinet meeting convened while Habib was still in Damascus that "certain measures have been taken to reinforce confrontation with the enemy" and the Cabinet emphasized in a statement that Syria considers itself "responsible for the defense of Lebanon and the Palestinian revolution against Zionist aggression."

At the same time, Prime Minister Menachem Begin of Israel told a cheering political rally that he will send warplanes to strike at the Syrian missiles deployed in Lebanon if Habib's efforts fall short of their removal.

"If they don't remove the antiaircraft missiles . . . an order will be given to the Israeli Air Force to act," Begin said, according to news agency dispatches from Jerusalem. "And when our Air Force goes to work, it gets results."

Habib, who is to hold talks with Israeli officials Monday, declined to discuss the substance of his nearly four hours of talks at the Syrian presidential palace. There was no detailed official Syrian comment except for the statement reiterating Assad's claims to defending Lebanon and the Palestians.

Although there often is a gap between official pronouncements and actual decisions in the Arab world, this and Begin's statement taken together left a strong impression that Habib had not budged the Syrians on Israel's insistence that the missiles must be removed from Lebanon's Bekaa Valley. Israeli officials queried in Jerusalem after Begin's remarks also said that to their knowkedge Habib has made no headway, the news agency dispatches said.

Against this background, the United States seemed caught in a major crisis focusing attention on the very Palestinian problem and Arab-Israeli conflict that the Reagan administration had hoped to postpone in favor of Persian Gulf security and anticommunism.

Habib said he and Assad had a "full exchange of views," a phrase sometimes used as polite way to express disagreement. But he added that he would not "discuss or characterize" the talks during his mission to Lebanon, Syria and Israel since "I want a full opportunity to review the positions of all involved in this situation."

The language of Syria's statement suggested that Assad was determined to send Habib off to Israel with a clear, if well-known Arab world theme: as Israel's major ally, the United States alone can, and should, stop what is considered in Damascus as the Jewish state's aggressive policies.

In addition, the tone of the Cabinet communique suggested that the Soviet-built AA6 ground-to-air missiles at the core of the crisis would remain in the Bekaa Valley where they were deployed after Israeli jets shot down two Syrian helicopters April 28 in central Lebanon.

The Soviet Union, which dispatched its first deputy foreign minister, Georgy Kornienko, to confer with Assad, was praised as "Syria's and the Arab world's best friend." Secretary of State Alexander M. Haig Jr. said last week that he believed the Soviets "are doing the best they can," but there was no sign here tonight that they were urging Assad to pull back the missiles.

Western and Third World diplomats in touch with the Soviets here and in Moscow reported that Kornienko, who ended a three-day visit here Friday, had expressed hope that a fifth-Israeli conflict in 33 years could be avoided. But they said he had backed Syria's positioning of the missiles in Lebanon.

Syria and the Soviet Union last October signed a friendship and cooperation treaty proviving for consultation and help in times of crisis.

Prime Minister Abdul Rauf Qasm presided over the special Cabinet meeting that reaffirmed Syrian responsibility for Lebanon and the Palestinians and adopted "certain measures to counter American-Israeli challenges" on that score.

"We are ready to face all eventualities; we have prepared ourselves and our plans," Qasm said according to the communique distributed by Syria's official news agency, Sana. "We possess the required power to hit the enemy in any of the battlefronts."

In addition to stressing the defensive umbrella over Lebanon -- which in the Palestinians' case has been unable to stop repeated Israeli air, artillery and ground assaults -- the Cabinet told Syrians to be on "full alert."

"All Arabs are required to observe national responsibility in standing beside Syria," the prime minister was quoted as saying.

One of goals of Syria's tough public policy in the crisis is to win back straying Arab world support for a government isolated by domestic problems and squabbles with its neighbors diplomats said.

The national news agency said the Cabinet had drawn up "comprehensive strategy . . . to thwart agression" and would push ahead with efforts to achieve a political settlement in Lebanon.

Syria has argued that it moved in the missiles only after Israel shot down the two helicopters, characterized here as resupply craft, to prevent an emerging Syrian-Lebanese Christian reconciliation that could have limited ability to exercise influence in Lebanon.