The rapid escalation in the United States military presence in or off the coast of Lebanon has led to deepening concern and criticism even among the closest Western and Arab allies, who have, until now, supported US policy in solving the Lebanese crisis.
The US still has only 1,200 marines on shore, based around Beirut's international airport. But the support complement stationed off the Mediterranean coastline has quietly but dramatically risen over the past month from 1,900 to 12,400 marine and naval personnel, according to a US Marines spokesman.
That brings the total to almost 14,000, more than one-third the size of the entire Lebanese Army. And it does not include the military personnel of the battleship USS New Jersey or its escort ships, which the Pentagon announced Monday would be joining the US Sixth Fleet in the eastern Mediterranean.
Beirut newspapers and Western envoys are speculating openly about the possibility of an increased US military profile, on the side of the Army, in the ''war of the mountains'' despite repeated denials from Washington.
The marines have already been dragged into the conflict, firing back from both land and sea at artillery emplacements established since Aug. 10 in the Shouf mountains overlooking marine positions. US warplanes and helicopter gunships have also been called into action, swooping low over the hills on reconnaissance missions. And US officials now say the marines have been authorized to call in air strikes from planes on the USS Eisenhower and USS Iwo Jima.
There is a growing feeling among many parties in Lebanon, including the three other contingents of the multinational peacekeeping force from France, Italy, and Britain, that the US particularly may be drawn in further if the Lebanese Army is unable to hold off the offensive by Druze Muslim militiamen.
Most of the Druze thrust has been aimed at the Christian Phalange militia in the Shouf. The Druze Progressive Socialist Party now claims to have pushed the Phalangists out of 42 villages in the Shouf, gaining control over 85 percent of the scenic region.
After 10 days of conflict, the Druze gunmen are also less than one-half mile from the coastal highway that links the capital with southern Lebanon.
But the Lebanese Army is also coming under attack on two fronts - at Khalde on the coast and Souk al Gharb in the hills - that are crucial to prevent the Druzes from either moving on the capital or hooking up with their Shiite Muslim allies in Beirut's southern suburbs.
For the third consecutive night, Souk al Gharb came under heavy attack four times Monday night and early Tuesday morning. An Army communique claimed the attackers had been repulsed, but in private military officials have told members of the multinational peacekeeping force that the Army cannot hold out indefinitely under the bad battering it is taking.
Should the Army lose Souk al Gharb, it is possible the mountain war would spread to the capital, making the conflict a full-scale civil war involving other leftist Lebanese factions. This would be ''a disaster'' for US-directed efforts to contain the battles and restore peace, according to a Western envoy.
Beirut's highly respected An Nahar newspaper, whose publisher, Ghassan Tueini , is leading the UN debate on Lebanon, said Tuesday that the Reagan administration is on the verge of taking firm action in support of the Army, providing substantial new military and logistical assistance.
Private and state-controlled radios in Lebanon interpreted the report as meaning that there would be an escalation in military action against the opposition forces to prevent shelling of the marines and to pressure the Druzes into negotiations.
But Radio Israel reported Tuesday the US was prepared to take even further action. Quoting sources in Washington, it said the Reagan administration had been sending messages that US warplanes might have to bomb Syrian positions in eastern Lebanon if the situation did not improve. The US and Lebanese governments have held Syria responsible for manipulating the war of the mountains because of its supplying and military backing of Lebanese opposition forces.
On the surface, the strengthening of the US military presence is designed to show the degree of the American commitment to finding a means for peace in this long-troubled land, while protecting the marines on shore. But Western military analysts predict it could end up being counterproductive, alienating forces the US is trying to lure to the negotiating table and spreading suspicions in the Arab world.
The unsophisticated Druze forces in the mountains widely believe the US is playing a major role in shelling them. Often baffled, they have asked reporters why the US is involved. In fact, the marines have been involved in limited actions, firing only when fired upon. The confusion may in part be due to the fact that both the Lebanese Army and the Phalangists use largely US or other Western-made weaponry.