Security Council members agreed informally tonight on a resolution that would have launched a limited inquiry into the Beirut massacre, but a meeting to approve the measure officially was postponed indefinitely by a bitter intra-Arab dispute.

Western and Middle Eastern diplomats reported that the Palestine Liberation Organization had threatened to go before the council and publicly accuse Lebanon of blocking a full-scale investigation to cover up the Lebanese role in the massacre.

The PLO thought the resolution was too weak. In the end, the Arab representative on the 15-nation council, Jordanian Ambassador Hazem Nuseibeh, pleaded that a debate tonight would produce "long polemical diatribes." He won the indefinite postponement, sources said.

The resolution, which at Lebanon's request would have barred any on-site investigation by a U.N. team, had been agreed informally by all council members after 10 days of arduous consultation.

The resolution would have called on the secretary general to appoint a special representative to "examine" -- rather than investigate -- the massacre of hundreds of Palestinians in two Beirut refugee camps "in a manner acceptable to the government of Lebanon."

In addition, the council would have invited governments and "interested parties" to "furnish any relevant data" on the incident -- which could encompass news reports and depositions from witnesses as well as the results of separate inquiries by Lebanon and Israel.

Despite the widespread demand for an international inquiry, reflected in a 149-to-0 vote by the General Assembly last Friday, tonight's resolution proved impossible to negotiate.

There was an unspoken awareness, council members conceded privately, that the Lebanese do not want to look too closely at the massacre because it was carried out by Christian militiamen linked to the current government.

Many council members sympathized with the Lebanese desire to avoid a process that would make it more difficult to unify the country -- a prime policy objective of the United States and others.

Secretary General Javier Perez de Cuellar also wished to avoid a direct role in the investigation for himself or his 50 peace-keeping observers in Beirut. "This would jeopardize their prime mission and could make them living targets," said one U.N. official.

But in the end the PLO -- which had insisted all along on an on-site investigation by a commission made up of council members -- was able to block the halfway measure that the council was ready to accept.

Israeli Ambassador Yehuda Blum said that Israel took no position on grounds that the massacre occurred in Lebanon and "it is for the Lebanese government to work it out with the U.N."