Can Beirut forces be safe?

As rescuers completed their search through the wreckage of the Marine complex in Beirut, the 1,800-man US contingent began converting its zone around the Beirut airport into a virtual fortress and deploying a new company of troops.

These moves reflect strong US fears of possible future attacks against the multinational force (MNF).

But there is also growing awareness about the difficulty of self-defense in Lebanon under the MNF mandate.

Some of the concern is based on warnings MNF sources say they have received over the past 10 days. At a rally Monday in eastern Lebanon, Hizbillah leader Sheikh Muhammad Yazbek repeatedly declared, ''Death of America,'' and pledged, ''We are determined to carry it out.'' Hizbillah (Party of God) is a pro-Iranian faction of Shiite Muslims, one of two groups suspected of having possible links to the bombings that killed 230 US and 58 French troops.

Although Mr. Yazbek has publicly denied responsibility for the bombings, he said, ''Let America, Israel, and the whole world know that we have a lust for martyrdom and our slogan is being translated into reality.

''America's fleets will not frighten us. We shall teach it a lesson it will never forget by our faith and strength. In our faith, we are stronger than Vietnam, and we shall deal strong and continuing blows.''

At the first of what is expected to be a long series of official US inquiries into the airport bombing, Gen. Paul X. Kelley, commandant of the Marine Corps, admitted to the Senate Armed Services Committee Monday: ''The US marines have been targeted for professional acts of terrorism and these will continue.''

The problem evolving is: What can the US do about it? A dispute appears to be growing over diplomatic policy vs. on-the-ground military planning.

An 11-person congressional delegation that visited the bomb site last weekend said the focus of the investigations would be whether security had been adequate , especially in light of the bombing of the US Embassy six months earlier.

''A lot of questions will have to be asked that one will not like to have to ask,'' Rep. Larry Hopkins (R) of Kentucky said.

One Marine officer resentfully labeled the inquiries ''head-hunting expeditions.'' He also predicted that ''some heads may roll, but that will not solve the basic problem.''

Officials from all four MNF contingents - the US, France, Italy, and Britain - said even before the attacks that the US mandate was unrealistic, especially in terms of self-defense. The mandate is often described as an effort to maintain peace in Beirut and to back up the government of President Amin Gemayel.

One badly injured marine put it succinctly: ''They should either pull us out or let us loose.''

''This mission has always caused heartburn,'' commented a survivor, who noted that the tour in Lebanon had been the hardest stint of his 18 years in the Marines, including Vietnam. ''This is a no-win situation.

''We have everything going against us. There are a lot of folks who don't want us here, and they use guns to say so. We are on the low ground, exposed on all fronts. We are here only to show the flag and to support the Lebanese government. We're not proper peacekeepers,'' a fact reflected in the absence of that crucial word from the US letter of understanding with Lebanon.

When the marines first arrived in Lebanon 14 months ago, they were not even allowed to load their rifles with magazines of ammunition without permission under the limited rules of engagement outlined by US mediators.

As fighting among rival militia groups escalated, the marines were authorized in recent months to use automatic rifles, mortars, artillery, and naval gunfire from warships lying off the Mediterranean coast. They have also made regular improvements, notably after the embassy blast, to their own positions.

''Sure, there was a lot more we could have done,'' conceded one officer. ''But you have to remember why we were sent here.'' He described the marine mission as an effort to cut down the psychological barriers and relax the atmosphere of fear built up during the previous seven years of strife. ''We didn't come here to hide, which is what we're having to do now.''

The side effects of the latest US and French security measures are indeed being felt in Beirut. New checkpoints and earthen barricades are found on main roads.

''This is no longer a living, breathing city,'' added a foreign resident.

At the same time, however, there are few alternatives. Expanding the marines' role would only sink the US deeper in the quagmire of messy Lebanese politics as well as alienate groups already suspicious of the superpower presence, say diplomats from the MNF countries. And withdrawing all but a token force onto Sixth Fleet ships would defeat the purpose of coming to Beirut in the first place, they add.

Yet to remain as they are positioned now leaves them open to assault. ''You really can't protect yourself completely from a suicide attack,'' said a marine sergeant. ''It's not only truck bombs you have to worry about. The guy could have divebombed us with a little plane full of explosives. It sounds outrageous, but so did a truck bomb two weeks ago.''