The United States joined tonight in a unanimous Security Council condemnation of Israel's occupation of West Beirut and in a demand for an immediate Israeli pullback from the Lebanese capital.

The resolution, worked out during day-long consultations among the U.S., Soviet, Lebanese and Jordanian ambassadors, also condemned Tuesday's assassination of Lebanese president-elect Bashir Gemayel and "every effort to disrupt by violence the restoration of a strong, stable government in Lebanon."

The text was no stronger in its criticism of the Israeli moves than the statements issued by the White House and State Department yesterday, but it was a rare example of American willingness to join in a condemnation of Israel at the United Nations -- the strongest since the response to Israel's attack on an Iraqi nuclear reactor in 1981.

Israeli Ambassador Yehuda Blum and his delegation left the council chamber shortly before the meeting began, in order to reach home by sunset for the start of the Jewish new year, Rosh Hashanah. He told reporters that he was "disappointed" by the American vote -- which had been foreseen -- because he considered the resolution "unbalanced." Nowhere, he said, "does it mention the root cause of the Lebanese situation."

Yesterday, Blum made clear to the council that Israel would ignore any U.N. demands and would leave Beirut only when the Lebanese Army is capable of taking over.

U.S. Ambassador Jeane J. Kirkpatrick remained silent throughout the two-day debate and made no public comment on the text following the 15-0 vote.

The Lebanese, in pressing for U.N. action, apparently did not expect that a council resolution would produce a response that a White House demand could not. Instead, Lebanon sought a token of American resolve and wished to increase pressure on Washington to force an Israeli withdrawal.

The Soviet Union sought to maintain that pressure by inserting into the resolution a requirement that the secretary general report back to the council within 24 hours on Israeli compliance.

The Soviets remained silent about the Israelis' temporary occupation in Beirut of parts of the Soviet Embassy compound. U.S. diplomats said they could have supported a Soviet request that the council condemn the violation of Soviet territory in the compound.

Other provisions in the council resolution included a determination that the Israeli occupation violates the cease-fire agreement negotiated by U.S. envoy Philip C. Habib, recognition of Lebanon's drive to "ensure the withdrawal of all non-Lebanese forces from Lebanon," and a call for respect of Lebanese sovereignty "under the sole authority of the Lebanese government and armed forces."

At the request of France, the council also included a provision calling on the parties to cooperate with efforts by the secretary general to deploy U.N. military observers to monitor the situation in the Beirut area, a process that Israel has thus far blocked.