A Palestinian leader accused by the United States of masterminding the hijacking of an Italian cruise ship was reported to have left here today for an unknown destination, frustrating the Reagan administration's attempts to bring him to justice.

Officials of the Palestine Liberation Organization said that Mohammed Abbas, 38, who flew to Belgrade Saturday night after being released by Italian authorities, was no longer in Yugoslavia. There was no independent confirmation of the PLO statement, and the officials refused to say where Abbas had gone or when he had left.

The Yugoslav decision to ignore urgent demands from Washington to arrest Abbas had been expected by western diplomats and political analysts here. It reflected the particularly close ties between communist but nonaligned Yugoslavia and Third World liberation movements such as the PLO.

By its action, Yugoslavia becomes the third country -- along with Egypt and Italy -- to strain its normally close relations with the United States in the aftermath of the hijacking of the Achille Lauro by four Palestinian gunmen last week.

In Rome today, Socialist Prime Minister Bettino Craxi held a lengthy meeting with his inner cabinet as dissension within his government grew over Italy's handling of the affair, including allowing Abbas to leave for Belgrade despite U.S. pleas to detain him.

In Cairo, Egyptian President Hosni Mubarak, also concerned about domestic outrage over the U.S. interception of an Egyptian airliner, demanded an apology from President Reagan.

The PLO announcement of Abbas' departure was carried without comment by the official Yugoslav news agency Tanjug.

In Washington, White House spokesman Michael Guest said that "we have no independent confirmation of Abbas leaving" Yugoslavia and that the White House had not received an official response yet to the U.S. request for extradition.

Belgrade officials privately justified their rejection of U.S. requests for Abbas' arrest by citing Article 202 of the Yugoslav constitution, which guarantees safe conduct or political asylum for "peoples fighting for the liberation of their countries."

In private, officials said that Yugoslavia hardly could be expected to detain Abbas and hand him over to the United States when Italy, a U.S. ally and member of the North Atlantic Treaty Organization, failed to act on a similar American request.

Yugoslav officials also contend that Washington has delayed or rejected several of their extradition requests despite a longstanding joint extradition treaty.

A spokesman for the PLO office in Belgrade, which enjoys diplomatic status, told journalists that Abbas was ready to go to the United States to explain the Palestinian position "if the Americans accept him as a representative of the PLO." In an interview with CBS News Sunday, Abbas said he would be willing to go to the United States if he is granted immunity from prosecution.

Abbas, who heads a faction of a PLO splinter group known as the Palestine Liberation Front, had been aboard the Egyptian airliner forced down in Sicily on Friday morning by U.S. Navy jets. Other passengers on the flight from Cairo included the four alleged hijackers of the Achille Lauro now in custody in Italy.

Describing Abbas as "one of the most notorious Palestinian terrorists," the White House has said it has strong evidence that he was "criminally implicated" in the hijacking of the Italian ship. This view is disputed by Yugoslav and Italian officials, who have stressed Abbas' role in negotiating an end to the hijacking. None of the White House evidence has been made public.

Abbas' presence in Yugoslavia was played down today by the state-controlled news media, which described him as "a member of the executive committee of the PLO." Most newspapers limited their coverage to a three-paragraph Tanjug dispatch, noting that the "successful" negotiations with the hijackers of the Achille Lauro had helped to avert "a great tragedy."

Political analysts here said that a prolonged stay by Abbas would have seriously embarrassed the Yugoslav government, which is anxious to preserve its good relations with the United States. The Reagan administration has led western efforts to help Yugoslavia, which occupies a strategic position between the eastern and western blocs, through a major debt crisis.

Yugoslavia has been criticized by Washington in the past for failing to do enough to apprehend international terrorists who have taken advantage of the country's "open-border" policy to travel undetected between East and West. Citizens of many communist and Third World countries can enter Yugoslavia without a visa.

It would have been easy for Abbas to slip out of Yugoslavia by car for a neighboring country today if he wished to avoid traveling by air.

There are PLO offices in Bulgaria and Romania. Like Yugoslavia, both countries recognize the organization as the sole legitimate representative of the Palestinian people.

One notorious incident involving Yugoslavia as a transit point for international terrorists occurred in 1976, when the West German government alerted Yugoslav authorities to the presence of the Venezuelan-born terrorist known as "Carlos." Carlos, wanted in France and Austria, was allowed to leave Yugoslavia quietly, despite calls for his arrest and extradition.

Abbas was traveling with a Palestinian colleague named Abul Ezz. Both men were allowed to leave for Belgrade.