A leading PLO official reiterated today that "we are ready to leave Beirut" but warned a special General Assembly session that an end to the siege of the capital would not resolve the Lebanese crisis until Israel withdraws fully and unconditionally.

Farouk Kaddoumi, who is the Palestine Liberation Organization's equivalent of a foreign minister, called on the United Nations to force Israel to withdraw by imposing a full range of sanctions upon it, or by rejecting the credentials of Israel's U.N. delegation.

He also endorsed a longstanding Soviet proposal for an international conference under U.N. auspices to discuss -- with PLO participation -- the broader Middle East dispute "on the basis of all U.N. resolutions on the question of Palestine."

The special assembly session, which began two years ago, was reconvened this afternoon just after Secretary General Javier Perez de Cuellar charged Israel with deploying a new force of Lebanese "irregulars" in a part of southern Lebanon under its control.

The group that Perez de Cuellar referred to has been cited in reporting from occupied Sidon as composed of Lebanese Christians sympathetic to the Israelis. It is in addition to the Christian militia headed by Saad Haddad -- which with Israeli backing has controlled an enclave adjacent to the border since the Lebanese civil war.

In a report to the Security Council, Perez de Cuellar said that at the end of June the new group, "equipped and controlled by the Israeli forces, attempted to establish checkpoints and patrol the villages."

The U.N. report went on to charge that "the ill-disciplined behavior of these irregulars, who are not recognized by the Lebanese government, led to friction with the other inhabitants." It said the U.N. peace force in southern Lebanon, UNIFIL, had tried to contain the activities of the group, which Israel calls a "civil guard."

The term of the 7,000-man U.N. force, which has had little to do since Israeli units overran its positions on June 6, expires on Thursday. The council is expected to meet Tuesday to extend its life for another 30 or 60 days -- as both the Lebanese government and Perez de Cuellar have recommended -- even though no role is envisaged for it under the peace-keeping plan being negotiated by U.S. envoy Philip C. Habib.

Israel reportedly has accepted the participation of yet another group of U.N. peace-keepers -- 10 military observers stationed at Yarze, southeast of Beirut -- in the PLO evacuation process that Habib is negotiating.

But U.N. officials said today that they had not been informed yet of Israel's acceptance, and that the 10 observers still are being prevented by Israeli troops from access to the front lines.

Last week the Security Council, with U.S. support, called on Israel to permit the deployment of the U.N. observers. But in this, as in all other endeavors to play a role in the Lebanese crisis during the last 10 weeks, the United Nations has been frustrated.

The reconvening of the assembly session today was part of an attempt by the PLO to take advantage of this frustration and reaffirm broad diplomatic support from the international community for the Palestinians' political aims.

Arab diplomats said a resolution condemning the Israeli invasion and reaffirming Palestinian rights would be put forward, intended to win the maximum number of votes and to isolate Israel and the United States in opposition. A vote is expected on Wednesday.

Both Arab and Western diplomats expressed doubts that the assembly action would either reinforce or interfere with the Habib mission -- unless Israel chooses to make an issue of the rhetoric or the voting.

Kaddoumi, in his speech, made no direct reference to the Habib talks, insisting that the PLO willingness to withdraw from West Beirut is intended solely to protect the city's civilian inhabitants. He denounced the U.S. mediating role as "flagrant hypocrisy and blackmail," saying Washington refuses to accept Palestinian rights or to enter into a dialogue with the PLO.

Egyptian delegate Amre Moussa charged that Israel is now engaged in the "dismemberment of Lebanon, partitioning its people and territory." He warned that there can be no peace negotiations until the PLO is recognized. Also, he said, the PLO must recognize Israel's right to existence, security, recognition and "the right to negotiate."