The United States escalated its involvement in Lebanon's mountain war between Druze-led militia and the Lebanese Army today as two American warships fired multiple volleys deep into what a spokesman said was Syrian-held territory to silence guns shelling the Defense Ministry and the U.S. ambassador's residence.

A Marine spokesman said the two ships, the destroyer USS John Rodgers and the frigate USS Bowen, opened fire on the guns late last night and early this morning because the shelling had "endangered American lives."

None of the shells had hit Marine positions at the international airport, making this the first time U.S. firepower has been used to protect Americans connected with the embassy and the Lebanese Defense Ministry.

A military spokesman in Damascus said tonight that Syria would strike back at American ships or Marine positions if fired upon again. "We emphasize that if any shell from land, sea or air falls in the areas where our forces exist, they will shell the sources of fire," Syrian television quoted the spokesman as saying.

Marine spokesman Maj. Robert Jordan said later that no return fire was detected after the Syrian announcement. He said the protective shield of the 12 warships and roughly 90 aircraft stationed off the coast now "extends to all Americans" in Beirut as well as the 5,400-man multinational peace-keeping force serving here and the U.S. diplomatic mission.

Later, he described it as covering all "other civilian and military personnel assigned to Lebanese government entities."

This was also the first time the U.S. Marine mission here has been defined in such broad terms. It appeared to open the door to an even greater American involvement in the fighting in the days ahead as the Army presses an offensive to expand its area of control in the mountains just east of the capital.

The Defense Ministry, where a number of U.S. advisers are working, and the area where the ambassador's residence is located have been shelled repeatedly from the mountains over the past two weeks without the Marines responding.

One reason is that the Army has its own gun positions all around the ministry. Ambassador Robert Dillon's residence is less than a mile below it and just above the presidential palace in the hills southeast of the capital.

The targeting by the Druze and their allies of the Defense Ministry and area around the palace is thought to be intended as a demonstration that the gunners are capable of destroying these symbols of government power.

Jordan refused to characterize the warships' firing as an escalation in the U.S. role in the war and said, "We are only responding to the threat with an appropriate defensive response."

But the response was far stronger this time than last week, when the Bowen fired four rounds from its five-inch guns after Marine positions at the airport were hit. One report said 60 rounds were fired this time, although there was no official confirmation.

The Marine spokesman indicated that American intelligence had concluded the threat to the 1,200 marines onshore had increased during the past few days because of the arrival of new long-range artillery in "Syrian-controlled territory" in the mountains.

"New elements have been introduced from farther away, requiring naval gun fire," he said. "There was long-range artillery from an area that has not previously threatened us."

Jordan described the fire from the five-inch guns on the two U.S. warships as "the deepest fire yet" into Syrian-held areas and said that subsequently the shelling "diminished considerably."

He refused to say who the marines thought was responsible for the shelling but noted that it was "obvious" that Syrian influence "has been extended into areas that it was not extended to before."

The spokesman offered few details, saying they might be useful to whoever was manning the batteries in the mountains.

A non-American western military source said the two warships fired repeatedly between 11 p.m. and 12:25 a.m. and that about 60 rounds had gone off. Later, a U.S. Embassy official said he thought "at least 30 shells, probably more" had been fired.

Also unclear was just how deep into "Syrian-controlled territory" the ships had fired. Syrian lines formally begin at Sofar on the Beirut-to-Damascus highway, perhaps five miles east of the main center of fighting, around the mountain resort town of Suq al Gharb.

The Druze and their various Palestinian and Lebanese allies have artillery behind Syrian lines as well as forward of them, making it difficult to determine who, as the Marine commander, Col. Timothy Geraghty put it, is "pulling the lanyard on the guns." But Jordan stressed that the fire had originated in "Syrian-controlled territory."

Another version of the target area of the two American warships came from the non-American military source, who said it was "north of Aitat," a Druze-held town just below Suq al Gharb.

If true, this would hardly qualify as Syrian-controlled territory--and would suggest the warships were firing at least partially in aid of the Lebanese Army there.

The Army has tried since yesterday morning to move south from Suq al Gharb along a ridge line to assure control of the greater Beirut area--and the access routes to it from Druze-controlled areas in the mountains.

More heavy fighting took place today around Suq al Gharb, with the Army reporting that its American-trained rangers engaged in hand-to-hand combat in the neighboring village of Kaifoun and gained control of most of it.

But reporters and photographers who reached Suq al Gharb today said the Army was still under shell fire there and busy clearing out snipers firing on its troops inside the town.

Three Lebanese Hawker Hunter jets flew several sorties today in support of the Army's offensive. Yesterday, the planes were used for the first time in eight years. One was shot down and another damaged among the five aircraft used. The Lebanese Air Force is believed to have started with only eight operational warplanes.

With the airport closed here, the aged Hawker Hunters have used a makeshift airstrip on the coastal highway.