Israeli jets shot down two Syrian helicopters in central Lebanon today and Prime Minister Menachem Begin warned of broader Israeli intervention in the escalating Lebanese conflict if the Syrian Army pursues its offensive against Christian militias in their mountain redoubts.

The downing of the two helicopters in eastern Lebanon's Bekaa Valley marked the first such publicly announced Israeli attack against the Syrians in direct support of Lebanon's rightist Christian militias. It raised the specter of wider fighting matching Israeli and Syrian forces and spilling beyond the conflict in Lebanon.

In ordering the attack on the helicopters and in his later comments, Begin seemed to be drawing a new "red line" against Syrian actions in Lebanon. He warned that Israel will not allow the Syrian Army to dislodge Lebanese Christian militias from the mountains northeast of Beirut -- the historic Maronite Christian heartland -- or to use helicopter gunships in its battles with the rightist irregulars.

"There are grounds to believe that it [today's action] will not suffice us . . . We do not want war with Syria, but we will not allow the Syrians to take over Lebanon and annihilate the Christians," Begin said in an interview on Israeli radio after visiting northern Israeli settlements. "It is unbelievable that a Jewish state will stand idly by and see such a thing taking place."

A Syrian government spokesman announced in Damascus that in addition to shooting down the two helicopters, Israeli planes strafed Syrian positions in the embattled mountains, Washington Post correspondent Jonathan C. Randal reported from Beirut.

The air strikes over eastern Lebanon appeared to have scuttled Syrian efforts to impose a truce -- and perhaps a longer-lasting settlement -- on the beleagured Christian militias. Diplomats and other analysts in Beirut were unsure whether Israel intended the strikes only as a warning to stop the Syrian offensive for total control of the crucial mountain ridges overlooking central Lebanon, or whether they also were designed to force Syrian forces to withdraw totally from the positions there that they now hold. s

Another possibility weighed in Beirut was new Israeli air strikes against other positions of the 22,000-man Syrian peacekeeping force. Even without the further strikes, it was unclear whether the Syrians could back away from a confrontation with Israel without loss of face for President Hafez Assad's isolated government in Damascus.

["Syria affirms it will resist with all determination any Israeli intervention of this sort," the Syrian statement in Damascus said, "and, at the same time, will maintain its efforts, already under way, to achieve national reconciliation in Lebanon."]

The Israeli Army command emphatically denied the reports of attacks on Syrian ground positions. It said the Syrian helicopters were knocked out because they were engaged in "an attack on and the murder of Christians."

Military sources said the first Syrian helicopter was shot down at noon by an Israeli F4 Phantom fighter-bomber as it was returning to its base at the Lebanese government's Rayaq airfield near famed Roman ruins at Baalbek in the Bekaa Valley. The second was downed about 5 p.m. by an F15, he added.

Begin said that if the Syrians control the Christian mountains overlooking the Bekaa Valley, "they can shoot at Jouneih, which is the only port through which the Christians can get supplies." Lebanon's main port in Beirut, about 10 miles south of Jounieh, has closed because of fighting in the capital.

"It is because of that that we got together this morning, consulted and took important decisions," the prime minister said.

When asked how Israel could force the Syrians out of Lebanon, Begin said, "It's not our job to get the Syrians out. And we will not send troops to get them out, but I had to make our position very clear."

He said Israel has kept the United States abreast of its decisions and that the Reagan administration understands Israel's motives for intervening.

[A State Department spokesman in Washington reiterated earlier appeals for all parties in the Lebanese conflict to exercise restraint but made no direct comment on the latest Israeli attack.]

The prime minister's warning appeared to remove at least some of the ambivalence in the government's views on how openly and at what level Israel should intervene in the worsening Syrian-Christian clashes.

Sharp differences of opinion over Israel's proper role in the conflict have been expressed within the Israeli Cabinet and among to-ranking generals in the Army and former general staff officers. On one hand, there have been strident statements from such Army officials as Maj. Gen. Avigdor Ben-Gal, commander of Israel's northern front, who said he would like to see "all-out Israeli support for the Christians now fighting with their backs to the wall, to enable them to liberate Lebanon from the Syrian conquerer."

In a controversial interview published in the Hebrew daily Davar, Ben-Gal said, "It is in Israel's interest to have a Christian Lebanon free of the Syrian Army."

Begin, who is also defense minister, has shown signs of applying restraint on the Army command, often through statements by his deputy defense minister, Mordechai Zippori. Frequently, Zippori has said direct intervention would drag Israel into a wider conflict, with no guarantee that the Syrians would be pushed out of Lebanon. Also, Army Chief of Staff Rafael Eitan recently said, "In my opinion, we must not be dragged into a war of attrition. rThe Syrians want attrition with cannons. Why should we operate in the sectors and with the means they choose?"

All the advocates of a moderate approach to the Syrians, however, have said that Israel should continue to strike aggressively at Palestinian guerrilla positions in Lebanon.

Correspondent Randal also reported from Beirut:

In apparent retaliation for Israeli air strikes Sunday and yesterday against Palestinian targets in southern Lebanon, Palestinian guerrillas shelled northern Israeli settlements. Israeli artillery and the cannons of Israeli-backed Christian militias in Maj. Saad Haddad's secessionist enclave fired back into Lebanon, according to reports from the border region.

Despite the Israeli attacks on their helicopters, Syrian forces continued their assault on Christian forces in the mountains and around the town of Zahle in the Bekaa Valley. Lebanese Defense Minister Joseph Skaff and Public Works Minister Elias Hrawi resigned to protest the heavy Syrian shelling of Zhale, their home town.

A U.N. spokesman reported, meanwhile, that an Irish solider serving with the United Nations in southern Lebanon has been shot dead and another Irish solider was missing after coming under fire from Palestinian guerrillas and their Lebanese allies.