Syria appeared today to be inching closer to more direct involvement in the Palestinian struggle to hold off invading Israeli columns in southern Lebanon. But it still had not committed fully its Air Force or regular infantry to the fighting despite numerous earlier pledges to do so.

This afternoon for the first time, two Syrian jets tried to intercept Israeli fighters that were attacking Palestinian strongholds around the capital, and one of the Syrian planes, a Mig 23, was reported shot down.

Syria said five of its soldiers belonging to the Arab Deterrent Force stationed in Lebanon were killed in clashes with the Israelis a few miles north of Nabatiyah, a Palestinian stronghold the Israelis now claim to have captured.

Reporters traveling to Rachaiyah at the southern end of Lebanon's Bekaa Valley said they saw Syrian artillery guns firing at Israeli positions 12 miles to the south.

Aside from these scattered incidents, however, there was no sign that Syria was preparing to commit its main forces to the battle. And News Analysis News Analysis there was every indication that the Israelis also were seeking to stay clear of the 30,000 Syrian troops here, including two new battalions that recently arrived.

Analysts here said Syrian President Hafez Assad was faced with an agonizing dilemma of how to create the appearance of coming to the aid of the Palestinians without putting his regular armed forces into the struggle at the risk of serious repercussions for his government at home.

With the Arab world now in a state of political disarray and preoccupied with the Iranian-Iraqi war and other issues, Syria is widely regarded as the only Arab country in a position logistically and militarily to come to the assistance of the hard-pressed Palestinians.

Egypt, probably the strongest Arab military power, remains a pariah because of its peace treaty with Israel, and President Hosni Mubarak has said he will not send troops to Lebanon.

Iraq is exhausted by its 20-month-old war with Iran, and the Arab states of the Persian Gulf are totally absorbed in efforts to contain that conflict and keep what they perceive as Iranian expansionism from upsetting their own vulnerable regimes.

Syria has long regarded itself as the foremost defender of the Palestinian cause in the Arab world and has pledged repeatedly to counter any Israeli invasion of southern Lebanon.

The irony of the situation is that for months leaders of the Palestine Liberation Organization and Syria have been discussing contingency plans for joint military action in case of the long-expected Israeli invasion.

What happened to all these plans remains unclear, but Palestinian sources report that the PLO and Syria remained at odds just before the outbreak of fighting on a number of political issues.

In late April, Syria and the PLO leader Yasser Arafat signed a document spelling out a common political and military strategy for dealing with Israel and other Middle East issues.

Reports at the time said the talks also included military planning for an eventual Israeli incursion into Lebanon.

As public proof of the Syrian willingness to come to the defense of the Palestinians, Assad sent planes up April 21 to interdict Israeli jets attacking guerrilla positions south of Beirut, and two reportedly were shot down.

Two other Syrian Mig fighters were downed May 25, when the Syrians challenged Israeli planes on a reconnaissance flight over the Bekaa Valley, where Syria has stationed the bulk of its 23,000 troops and ground-to-air missiles.

Thus, the absence of Syrian jets in the skies over Lebanon until the end of the fourth day of Israeli pounding was conspicuous. Yesterday, a military communique from Damascus said Syria was holding back its Air Force for "strategic military reasons."

There is still a serious risk that Syrian troops by accident or design may become involved in the fighting as the Israelis push inland from the coastal city of Sidon toward Jazzin, where a large number of Syrian troops are stationed.

For Assad, a past master of brinkmanship with Israel, the present crisis is reminiscent of the last Israeli invasion of southern Lebanon in March 1978, when the Syrians also did little to help the Palestinians.

Then, as now, Assad and other Syrian officials repeatedly pledged to come to the Palestinians' rescue prior to the invasion, although the latest promises were even more explicit than earlier.

Already, the Palestinians here are showing a mixture of bitterness and resignation at the recurrence of the same scenario, mindful that Assad's caution in tangling with the Israelis, or even coming to the aid of the Palestinians in other times and crises, is now legendary.

No one here or in Syria has forgotten that Assad was commander of the Syrian Air Force in September 1970, when the Syrian Army crossed into Jordan in an abortive bid to help Palestinian guerrillas then fighting the Jordanian Army in Amman.

Syrian tanks and infantry took a beating because Assad refused to commit the Air Force to their protection.

That Syrian debacle triggered the events that led to Assad's coup against then-president Nourreddine Attasi and his subsequent rise to power.

There may be other reasons for Assad's caution in the current circumstances, however.

One reason almost certainly is Assad's preoccupation with his bitter feud with Iraqi President Saddam Hussein, who is now in deep trouble as a result of recent Iraqi reverses in its war with Iran.

It is no secret that Assad would dearly like to see Saddam Hussein overthrown. But involvement in the conflict in Lebanon would only serve to turn Syrian attention away from the Iraqi front and could serve to undermine Assad's power just as the war with Iran has done to Saddam.

Another factor in Assad's calculations is likely to be the Soviet reaction to a deeper Syrian involvement in the fighting inside Lebanon.

According to a two-year-old friendship and cooperation treaty between Moscow and Damascus, the two governments are committed to consult one another whenever there is a threat to peace in the region.

The Soviets are also obliged by the treaty to help Syria defend itself against external aggression, but the accord does not cover Syrian troops or missiles stationed in Lebanon.

Earlier this year, Assad had pushed Moscow to expand its protection of Syria to include forces based in Lebanon, arguing that it constituted the country's front line of defense against Israel.

Discussions on this issue were held in Moscow in January, but nothing was ever made public to indicate that the Soviets had accepted the Syrian request. Most reports indicated that the Soviets were highly reluctant to immerse themselves in the Lebanese quagmire and had rejected the idea of expanding its obligations to Syria.