The United States, which has moved increasingly away from supporting unconditional Israeli withdrawal from Lebanon, has informally told members of the United Nations that this is no longer a realistic avenue for resolving the crisis.

A senior U.S. official at the United Nations said this view had been conveyed yesterday in informal consultations with other members of the Security Council. He said, "The situation has moved beyond Resolution 509," a reference to the council's unanimous June 6 demand that Israel "withdraw all its military forces forthwith and unconditionally."

The official also said the United States has not decided whether to support a continuation of the U.N. peacekeeping role in southern Lebanon. That role is due to expire Saturday.

These developments came as Secretary of State Alexander M. Haig Jr. met with Egyptian Foreign Minister Kamal Hassan Ali. According to Egyptian diplomatic sources, Ali counseled Haig against linking the withdrawal of Israel from Lebanon with that of Syrian and Palestinian forces.

Nevertheless, Haig in a television interview Sunday, and other officials in public statements yesterday, appeared to be supporting such conditions for the Israeli withdrawal.

The U.S. official at the United Nations, who asked not to be quoted by name, said that the demand for unconditional Israeli withdrawal is "no longer adequate to the needs of the situation." He pointed instead to the activities of special U.S. envoy Philip C. Habib, who was in Beirut yesterday seeking a basis for strengthening the Lebanese government.

State Department spokesman Dean Fischer yesterday denied that instructions have gone out to inform any other nations that Resolution 509 is no longer relevant. "We stand by the U.N. vote," Fischer said.

At the same time, a State Department official said that in order to achieve an Israeli withdrawal "it is necessary to work within the realm of the possible." This evidently referred to Israel's position that it will not withdraw its forces until it is assured of the pullout of Syrian and Palestinian military forces from Lebanon.

Fischer, asked where the United States expects the Palestinian fighters to go, said yesterday that "Palestinians who remain in Lebanon would have to be subservient to the central government." At another point, he told reporters that Palestinians remaining in Lebanon should be prepared to "accept the authority" of the Lebanese government.

A diplomatic source familiar with the current discussions said there is talk that any Palestinian guerrillas remaining in Lebanon would not be permitted to acquire heavy weapons such as artillery and rockets.

One major topic of discussion at high levels of the administration is how to meet Israel's demand for international guarantees of a demilitarized southern Lebanese buffer zone. The Jewish state, which is distrustful of the current 7,000-man U.N. force in Lebanon (UNIFIL), is reported to be urging the creation of an international group, outside of U.N. control, which would include combat troops from the United States.

This problem is believed to have been discussed at the White House late yesterday by the National Security Council, with President Reagan presiding. The meeting was held as Reagan prepared to dispatch Vice President Bush as his representative at a memorial service for King Khalid of Saudi Arabia, who died Sunday.

The on-again, off-again cease-fire in Lebanon was the topic of continuing diplomatic activity over the weekend and again yesterday, as both the United States and Egypt, among several countries, reportedly urged Israel to show restraint in its military activity and specifically not to invade Beirut. Haig said early yesterday afternoon that "we have been assured that Israel has no intention of occupying Beirut." He added that the United States "anticipates" that Israel would not take such action.

As Haig said a public goodbye to Ali after nearly three hours of discussion, the two men agreed that the Israeli invasion of Lebanon will require at least a temporary delay in Israeli-Egyptian negotiations for Palestinian autonomy on the West Bank and Gaza Strip.

Ali, while saying that the peace process with Israel is "a fact" and "valid," said that "some time" will be needed to overcome current difficulties. In private, according to informed sources, the Egyptians expressed the hope that an early agreement on Israeli withdrawal from Lebanon will make it possible to discuss resumption of the autonomy negotiations in about a month.

Haig, for his part, said it would be difficult to continue with the autonomy negotiations until the situation in Lebanon "has been clarified."

He added, "It would be our hope that the ultimate solution to the Lebanese crisis will be a catalyst for facilitating progress in the peace process rather than an obstacle to progress." Officials said he evidently had in mind the positive effect of an improvement in Israeli security that would flow from a more stable and permanent solution in Lebanon