American military leaders have become increasingly apprehensive about the presence of U.S. Marines in Lebanon, believing the situation on the ground there is like a time bomb that could explode at any time.

Marine Corps Commandant Gen. Robert H. Barrow, according to those who know his thinking, wants to get Marines off the ground there soon, even if it means putting them on ships off Beirut where they could await emergencies.

Barrow, sources said, feels the longer the Marines are kept in the twilight zone of military rules present in Beirut, the greater the chance that there will be an accident, like a shootout between Marines and some other forces.

Also, the commandant is said to be worried that this second long stay in Beirut will discourage reenlistment and soon affect the corps' worldwide deployment capability, leaving other possible world trouble spots without adequate cover.

The Marines went into Lebanon for the second time in 1982 on Sept. 28. The 1,200 on the ground now and the 600 on five ships off Beirut are scheduled to be relieved by a fresh force from Camp Lejeune, N.C., in mid-February, the 22nd Marine Amphibious Unit.

Administration leaders predicted last October that all other foreign forces would be out of Lebanon by year's end, which would clear the way for the Marines also to leave. But stalemated negotiations to bring about the withdrawal of Israeli, Palestine Liberation Organization and Syrian forces make the Marine presence look indefinite at this point.

Civilian policy planners at the Pentagon said they understand the military's concerns about the Marines' seemingly open-ended stay in Lebanon but that they know of no graceful way to withdraw any until Israeli and Syrian troops start leaving.

Rather than focusing on withdrawing, said one Pentagon civilian, the planning at the moment is how best to increase the Marine presence in Lebanon by one battalion if the United States decides to help expand the present multinational buffer force as desired by Lebanese President Amin Gemayel.

Asked if it would not make sense to substitute Army troops for the 1,200 Marines in Lebanon now that the American military's stay looks open-ended, one administration official replied, "It's true that the Marines are not in their usual role, but we're still thinking more Marines. We still believe there will be a break in those negotiations soon."

Marines traditionally act as assault troops, not policemen working under all kinds of restraints, as they are doing in Beirut. But the Marines keep the American military profile in Lebanon much lower than it would be if Army forces were substituted, officials said.

This is because the Marines' big supply base is on the five ships standing off Beirut and virtually out of sight. The Army would have to build a highly visible support base if soldiers were sent into Lebanon to replace the Marines.

"The Army already has a battalion on the Sinai," said one official. "I'm sure Army Chief of Staff E.C. Meyer doesn't want to send troops into Lebanon as well." There is no enthusiasm for the Lebanese mission by any member of the Joint Chiefs of Staff, Pentagon civilians conceded.

The Marines in Lebanon have been largely confined to patrolling near their base at the Beirut airport and training Lebanese troops in the field to use American equipment. The Pentagon plan to send Marines out on the Beirut-to-Damascus highway is still alive, officials said, but has been put on hold until the Israelis and Syrians start leaving Lebanon.

"Israelis man checkpoints on the highway outside Beirut," said one official, contending it would be asking for trouble to send the Marines through them before the Israelis abandon them. "We're not there to fight Israelis," said a Marine general.

However, once the Israelis and Syrians get off the Beirut-to-Damascus highway, an objective of the negotiations, plans call for the Marines to move onto it, probably occupying some of high ground alongside to provide additional reassurance to Lebanese forces.

Pentagon officials said they still are optimistic that there will be 16 "adequate," American-equipped Lebanese battalions ready for duty by the end of February. "We believe those battalions would be able to move out if the Syrians and Israelis leave," said one administration executive.

Lt. Gen. Philip C. Gast, director of the Pentagon's Defense Security Agency, went to Lebanon this week to assess the ability of the Lebanese military forces to absorb equipment already in country or on the way, including 124 armored personnel carriers, 300 trucks, 34 M48 tanks and radio gear. Gast is slated to return to Washington with his report this weekend. Congress, like Barrow and fellow members on the Joint Chiefs of Staff, is becoming increasingly apprehensive about the length of the Marine stay in Lebanon but has not gone beyond warning President Reagan that it could "direct the removal of such forces at any time they are engaged in hostilities." CAPTION: Picture, Bulldozers begin clearing destroyed buildings and rubble from war-ravaged downtown Beirut, where the Lebanese government is planning an $80 million rebuilding project. AP