Mohammed Abbas, a leader of the Palestine Liberation Organization whose arrest Washington had sought in connection with the hijacking of an Italian cruise liner, slipped out of Italy tonight aboard a Yugoslav airliner, according to U.S. and Italian officials.

In the latest turn in the Achille Lauro hijacking drama, Abbas and a man thought to be another senior PLO official fled Rome's Leonardo da Vinci Airport before Italian officials acted on the U.S. arrest warrant and were later reported to have arrived in Belgrade.

"I'm not happy about what happened here today," U.S. Ambassador to Italy Maxwell Rabb told reporters after a meeting with Italian Prime Minister Bettino Craxi.

Italian officials said Rabb had delivered a stiff protest to the Italian government during the meeting. U.S. officials have indicated privately they believe the Italians allowed Abbas to leave the country, although Italian officials maintained tonight that Abbas had boarded the Yugoslav airliner under a false name.

Abbas is a member of the PLO Executive Committee and the head of a faction of the Palestine Liberation Front, to which the hijackers of the cruise ship said they belonged. A U.S. citizen, Leon Klinghoffer, 69, of New York, was killed in the hijacking.

The four suspected hijackers of the liner, who were formally served with Italian arrest warrants today, told investigating magistrates that the hijacking of the ship was precipitated when a waiter discovered them cleaning their weapons in a cabin, Italian judicial sources said. The waiter tried to flee to tell the captain there were armed men aboard, and the hijackers reportedly decided to seize the ship. They reportedly told the magistrate their original plan had been to get off the ship at the Israeli port of Ashdod and attack there.

The four suspects were jailed in Syracuse, Sicily, pending transfer to another high-security prison on the Italian mainland.

Judicial sources said "about 10" Americans who had been passengers on the Achille Lauro were flown to Syracuse to identify the suspects.

Francesco Meloni, the chief prosecutor of Genoa, where the cruise began, said not all the Americans were needed to testify, because "the identification procedure by some of the Americans was sufficient."

The suspects, Abbas and another unidentified senior PLO official were brought to Italy early yesterday when four U.S. Navy F14s from the carrier USS Saratoga intercepted an Egyptair jet carrying them and forced it to land at the NATO air base in Sigonella, Sicily.

The suspects apparently were seeking safe haven, but the plane already had been refused permission to land in Tunisia, where the PLO has its headquarters, and in Greece.

Abbas and the other PLO official were interviewed yesterday at Sigonella by Italian magistrates in connection with their mediation of the hijackers' surrender so the Italian magistrate could establish that the four youths on board the diverted airliner were the hijackers.

Because another Italian prosecutor investigating other Arab terrorist activities here wanted to talk to Abbas also, according to judicial sources, he, and the Egyptian plane, were not allowed to leave Italy last night as Craxi had said they would.

Instead the Egyptian airliner and its 19 remaining passengers -- 17 Egyptians and the two PLO officials -- was flown late last night to Rome's Ciampino Airport, so deputy state prosecutor Franco Ionta could interview Abbas.

While that interview was taking place here today at the Egyptian Academy cultural center, where the PLO leaders were staying, U.S. District Judge Charles Richey in Washington issued an arrest warrant for Abbas, as the leader of the Palestine Liberation Front, on charges of piracy, hostage-taking and conspiracy in connection with the hijacking.

A U.S. Embassy spokesman here said tonight that a formal extradition request based on that U.S. arrest warrant had been forwarded to Italian officials "during the course of the day, early on." But in Washington, officials indicated that the FBI was still working on the tentative extradition plans.

Abbas mediated in the hijackers' surrender after Prime Minister Craxi asked PLO leader Yasser Arafat for help in ending the hijacking, which began Monday off the Egyptian coast. Arafat had denounced the hijacking from its inception and denied any PLO involvement.

Tonight, before a major dispute could develop over the U.S. extradition request between Italy and the Arab world it has sought to court, Abbas apparently left the country.

The events that led to his apparent escape began when the grounded Egyptair Boeing 737 suddenly took off from Ciampino Airport and flew to Leonardo da Vinci Airport, Rome's main commercial air terminal, about 10 miles away.

Authorities at da Vinci Airport said two men of dark-complexion, assumed to be Arabs, descended from the Egyptian airliner and quickly made their way to a waiting Yugoslav JAT Airways jet that had been delayed on the ground for one hour and 50 minutes from its 5:20 p.m. scheduled departure time. Minutes after the two men were seen to board the JAT airliner, it took off for Belgrade.

Italian sources here said they believe the two men who boarded the JAT airliner were Abbas and the other PLO official, who was unidentified. Craxi said in a press conference last night that the man was another member of the PLO Executive Committee and a close adviser of Arafat.

Associated Press reporters at Belgrade Airport reported that Abbas and his aide were allowed to bypass normal customs procedures, were met by PLO officials and driven away in a limousine bearing license plates of the PLO mission.

Abbas did not talk to reporters, but a passenger on the airliner said he had talked to Abbas during the flight and that Abbas had told him he would be in the Yugoslav capital "for two or three days . . . for a brief rest."

The nationalities and exact identities of the four Arab youths being held as the hijackers are still subject to some official conjecture because they were all apparently carrying false passports.

The pirates surrendered to Egyptian authorities Wednesday after mediation by officials from Egypt and the PLO, which reportedly was to take custody of the hijackers after they were promised safe passage from Egypt.