President Reagan has authorized marines in Beirut to call in air strikes against forces shelling their positions, serving notice on Syria that the United States is ready to escalate its firepower in Lebanon, White House officials said last night.

Although the authorization may increase pressure in Congress to invoke the War Powers Resolution, White House officials told Democratic House leaders yesterday that Reagan remains opposed to invoking a section of the act that could require withdrawal of U.S. forces from Lebanon in 60 to 90 days.

"We don't want to have our hands tied that way," a senior administration official said.

The decision allowing the marines to call for air strikes was made by top administration executives and representatives of the Joint Chiefs of Staff in a series of meetings over the weekend, sources said. The strongest argument for additional military action, they said, came in communications from Robert C. McFarlane, Reagan's Middle East special envoy.

His emphasis, officials said, was not only that the marines must be given more protection by having authority to call in their AV8 Harrier fighter-bombers and Navy A6 bombers from the carrier USS Eisenhower off Beirut, but also that the U.S. government must demonstrate to Arab nations that it is a reliable ally.

Among those involved in the weekend meetings were national security affairs adviser William P. Clark, Secretary of State George P. Shultz, Secretary of Defense Caspar W. Weinberger and Army Gen. John W. Vessey Jr., chairman of the Joint Chiefs, officials said.

The president approved their recommendations, an official said, and agreed that marines in Beirut could ask for help from Marine and Navy fighter-bombers without always going up the chain of command to Washington.

After the meeting on the War Powers Resolution, White House officials said they were receptive to alternative congressional action that would put an 18-month limit on the mission of the marines in Lebanon and restrict their role in the combat there.

The White House meeting was attended by chief of staff James A. Baker III, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and other Democratic congressmen.

At the State Department, spokesman Alan Romberg said there is "increasing evidence" that Syrian-backed units of the Palestine Liberation Organization are taking part in the heavy fighting around Beirut.

Senior department officials said privately that there is no longer any doubt that PLO forces are fighting alongside Lebanese factions working against President Amin Gemayel and that Syria has been providing arms and logistical assistance.

White House officials said that congressional leaders had been supportive privately of administration actions in Lebanon, but also said they believed that Reagan, under a provision of the 1973 War Powers Resolution, is required to inform Congress that U.S. troops are involved in a situation "where imminent involvement in hostilities is clearly indicated by the circumstances."

Once the president took such an action, he would be obliged to withdraw U.S. forces within 90 days unless Congress during that period approved keeping the marines on station. Putting such a clock on the mission "would send the wrong signal--to our troops, to our allies and to the Syrians," said one administration official.

Another official said this also would be likely to touch off a domestic political debate that would complicate U.S. diplomatic efforts in the region.

U.S. officials say invocation of the 90-day provision would be "a wrong signal" in that it would encourage the Syrians to hang back from diplomatic efforts to end the fighting in Lebanon. Some officials said that the Syrians would be likely to increase their support of Gemayel's opponents in the hope of drawing U.S. marines into combat and increasing U.S. domestic pressure for withdrawal.

These officials said that a presidential signal of withdrawal also could be harmful to the morale of the marines and might encourage the French, who also have troops in Lebanon that have taken casualties, to reconsider their role.

According to administration officials, House Foreign Affairs Committee Chairman Clement J. Zablocki (D-Wis.) and O'Neill said they believed the "imminent hostilities" section of the War Powers Resolution should have been invoked Aug. 29, the day two marines were killed in Lebanon.

White House strategy is to seek a compromise that would demonstrate congressional support for the U.S. peace-keeping effort in Lebanon, without yielding on the administration's basic legal position that the disputed section of the War Powers Resolution is inapplicable.

Officials said the White House is willing to accept a congressional initiative, also provided for in the War Powers Resolution, that would restrict the mission and role of marines in Lebanon but could not force their withdrawal within the next few months.

"We're seeking bipartisan support, not a Tonkin Gulf resolution," said one White House official, in reference to the unlimited declaration of congressional support won by President Johnson. That resolution became the basis for broadscale U.S. participation in the Vietnam war.

This official said that a resolution expressing support for administration objectives but imposing an 18-month limit would be "beneficial" from the point of view of persuading the Syrians that Reagan's deployment of marines has U.S. domestic backing.

What administration officials are most anxious to prevent, a concern they say is shared by some congressional leaders, is a free-for-all debate in Congress that would be interpreted as a sign of U.S. weakness.

The consultations yesterday were conducted by a White House team of Baker, presidential assistant Richard G. Darman and chief of legislative liaison Kenneth Duberstein with O'Neill, Zablocki and House Majority Leader James C. Wright Jr. (D-Tex.). They are scheduled to continue today with other members of Congress.

The negotiations came amid what U.S. analysts say they believe is a stepped-up Syrian effort to bring down the Gemayel government.

State Department officials said that, while the evidence is not conclusive, there are some grounds for believing that the groups attacking the Lebanese Army include some Syrian troops, wearing the uniforms of the irregular militias, and some Iranian revolutionary guards known to be in Lebanon to encourage pro-Khomeini sentiment among the Shiite Moslem population.

These officials said the Palestinian fighters involved appear to be drawn both from Syrian-sponsored Palestinian guerrilla groups that have kept separate from the PLO and from PLO units that recently rebelled against the leadership of Chairman Yasser Arafat and have come under Syrian control.

The involvement of Palestinians has been confirmed through interception of messages and identification of fighters killed in weekend clashes with the Lebanese Army, these officials said.