The United States vetoed a Security Council resolution tonight that threatened Israel with sanctions unless it agreed within six hours to a cease-fire and an unconditional withdrawal from Lebanon.

The other 14 council members voted for the resolution, put forward by Lebanon, which condemned Israel for rejecting previous U.N. cease-fire appeals.

American Ambassador Jeane J. Kirkpatrick explained that the sanctions threat and the condemnation made the resolution too unbalanced to "accomplish the objective of ending the cycle of bloodshed and violence and establishing peace in Lebanon."

She said the United States is now "engaged in every possible effort to bring the violence to an end."

But the Palestine Liberation Organization delegate, Zehdi Labib Terzi, charged the United States with being "concretely" involved in the Israeli drive against Palestinian forces in Lebanon.

"It is the United States in collusion with Israel who is attacking us," he said. Terzi and other Arabs threatened earlier in the day to take the crisis in Lebanon to a special session of the General Assembly, where no veto power exists and a majority could oust Israel from participation.

Lebanese Ambassador Ghassan Tueni, however, took the opposite tack, praising the United States for its earlier demands that Israel withdraw and expressing confidence in American efforts to obtain this objective. He played down the possibility of taking the crisis to a special session of the General assembly.

Earlier, Israel had rejected council demands that its troops withdraw unconditionally from Lebanon, saying a pullback was "inconceivable" without "concrete arrangements" to permanently safeguard Israeli citizens.

Israeli Ambassador Yehuda Blum defended the Israeli invasion as "the exercise of the inalienable right of any state to defend itself," in this case against Palestinian attacks on Israeli citizens, for which, he said, Lebanon had become the "launching pad."

Secretary General Javier Perez de Cuellar reported to the council that members of the 7,000-man U.N. peace-keeping force in southern Lebanon, known as UNIFIL, had remained in position and attempted to block the Israeli advance "despite constant danger to their lives," until "their safety was seriously imperiled."

He said Dutch troops had planted obstacles in front of advancing Israeli tanks, damaging one. And a Nepalese unit blocked a key bridge until its position was destroyed this morning, he said.

PLO representatives have charged that the U.N. troops had failed to block the Israeli advance by force of arms. Perez de Cuellar also relayed UNIFIL reports that the port of Tyre had suffered extensive damage and fires and that most of its inhabitants were now clustered on the beach.