If American troops are put ashore in Lebanon as part of a peace-keeping force, they will be used as a "screen" to help Palestinians leave the country and to "safeguard and secure" checkpoints, embarkation points and depots where the departing guerrillas could leave their weapons, Defense Secretary Caspar W. Weinberger said yesterday.

He also indicated that the U.S. troops would have a role in keeping the Palestinians and Israelis apart. He told reporters at a breakfast meeting that at the end of a military conflict, when sides are retreating or leaving their positions, they are "extremely vulnerable" and need some kind of "rear guard."

American military officials have said privately that one of the riskier elements of any peace-keeping operation would be the temptation of either side to make a surge at the other during this time of maximum vulnerability.

There is also some private concern here that the Palestinian leadership might not be able to restrain guerrillas from attacking U.S. peace-keeping forces because of general American support for Israel.

It is expected that the peace-keeping force would take up its position before any withdrawal begins.

Weinberger acknowledged that it is "risky business" to put American troops into Lebanon, but he said the risk had to be weighed against "the desire that the United States not be the cause of things not working out" by standing aside from such a role and letting the war be carried on a "disastrous" course into heavily populated west Beirut.

Weinberger stressed that many conditions would have to be met before American troops were put ashore and that there is obviously no agreement by all sides to any plan. He said the idea was to work out an agreement that would minimize the risks to American and other peace-keeping forces while avoiding further civilian casualties from Israel's battle with the Palestine Liberation Organization in Lebanon.

White House and State Department officials yesterday said the negotiations in Beirut between U.S. special envoy Philip C. Habib and a host of interested parties were at a "very sensitive stage" and were continuing despite a partial rejection of American involvement by PLO leader Yasser Arafat.

Arafat was reported to have rejected an evacuation of Beirut under American "protection" but to have left left open the prospect of U.S. involvement in a force separating the Palestinians and the Israelis.

White House spokesman Larry Speakes suggested that the public PLO reaction not be taken at face value. "We would be hopeful that all parties would go along," he said, adding, "I would look very carefully at statements made by the PLO before I would make a conclusion."

Speakes said the United States has urged Israel to re-open the flow of drinking water to the guerrilla enclave in west Beirut. The Israelis reportedly restored water and electricity for the first time in four days, but kept up a fuel and food blockade to maintain the pressure on the PLO forces.

In his meeting with reporters Weinberger repeatedly came back to the toll that Israel's invasion has taken on civilians. While declining to condemn Israel directly, his remarks reflected displeasure with the actions of the government of Prime Minister Menachem Begin.

Weinberger said the American offer of troops under certain conditions was an attempt "to prevent a massacre" that would occur if there were a full-scale Israeli attack on remaining PLO strongholds in west Beirut.

Asked if American interests have been advanced or hurt by the Israeli sweep through Lebanon, Weinberger said, "I don't think it's proper to say American interests are ever advanced by unilateral resort to war."

Although it has long been an American policy goal to have secure borders for Israel, and there have been problems of PLO shelling of Israeli border villages, Weinberger said he didn't think the unilateral resort to force "advanced the interests of the U.S. or its allies . . . or the free world," and that it "certainly has resulted in civilian casualties, deaths and homelessness to people who had nothing whatsoever to do with the issues involved."

Furthermore, Weinberger said he thought Israel's actions had created "an example or precedence problem" and "I . . . hope that we don't have further resorts to war."

He rejected the idea that by dispatching peace-keeping forces the United States may be helping the PLO to survive and, in effect, may be dealing with it as an organization.

The other side of that argument, Weinberger said, is that not trying to give Habib all the help he needs would be, in effect, to "assist or acquiesce" in further military actions that could hurt "a great many people" who are not PLO members.

President Reagan agreed, in principle, to the possible use of American troops in a peace-keeping force at the request of Habib to give the special envoy greater flexibility in trying to help the warring sides reach a settlement and to get the PLO out of west Beirut. Weinberger said Habib needed that flexibility in fast-moving and intricate negotiations.

Weinberger said it was his understanding that the Israeli cabinet had set a deadline of next Sunday for putting together an agreement that would get the PLO out of Lebanon.

But a report on the Israeli defense forces radio yesterday said Israel has informed Washington that it is ready to allow more time. A State Department official said he knew of no deadline.

Before any troops are sent in, and to minimize the risks to those troops, Weinberger said several conditions would have to be met.

Assurances would be necessary from authorities of all groups and governments involved that the peace-keeping force would be protected, accepted by all parties, invited in by the government of Lebanon, and that other countries--he mentioned France--would participate in the force.

The fact that there are sometimes no reliable spokesmen for disparate Palestinian groups is a big problem, Weinberger said.

In this country, he said, the president would want full understanding from Congress and "some kind of acquiescence."

The mission, Weinberger said, would be "very limited" in scope and time, probably 30 days or less.

Yesterday, three amphibious ships of the U.S. Sixth Fleet linked with a helicopter carrier and another assault vessel about 150 miles off the Lebanese coast. While there are 1,800 Marines with the fleet, Pentagon sources say they are not necessarily the men who would be part of the force. Army troops airlifted from Italy or the United States could be used, they said.

Weinberger said details of where the Palestinians would go and on what kind of transportation remain to be settled, assuming that an agreement can be worked out. He also indicated that the question of what happens to PLO weapons is a major one.

Pentagon sources say a plan is being discussed in which either the PLO would move equipment to storage areas under control of the Lebanese government or allow the Lebanese to do the moving.

Meanwhile, the chairmen of two key House subcommittees said yesterday that they support the plan for a temporary U.S. peace-keeping force.

Rep. Lee H. Hamiliton (D-Ind.) of the House Foreign Affairs subcommittee on Europe and the Middle East lent his support provided certain assurances were obtained that all parties want the U.S. troops there. The other vote of confidence came from Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House defense appropriations subcommittee.

The National Council of Churches yesterday launched a $1 million campaign for relief aid for Lebanon, and council officials criticized Israel for "needlessly obstructing" distribution of crucial supplies by delaying their shipping, unloading, documentation and distribution.

The American Lebanese League, which represents Americans of Lebanese origin or descent, also announced its "full and complete" support the decision.

In a news conference here, Robert Basil, chairman of the organization's policy committee, called Reagan's decision "a judgment call," but said the risks involved in sending forces are smaller than the risk of not doing so.

Basil said the previous policy of tolerating the PLO in Lebanon in an effort to contain it there had proved to be disastrous for Lebanon and the Mideast.