Palestinian hijackers holding more than 420 persons hostage on an Italian cruise ship in the eastern Mediterranean claimed today to have killed two of their American captives, according to monitored radio transmissions, but a later broadcast from a man claiming to be the ship's captain made no mention of any violence.

In Washington, State Department spokeswoman Elizabeth Thornbill said, "There is no reason to believe that these reports are true . . . we have had no direct contact with the ship."

The hijackers, in the broadcasts, threatened more killings if their demand for the release of 50 Palestinian prisoners held by Israel -- and possibly others elsewhere -- was not met. They identified themselves as members of the Palestine Liberation Front, a small group that has splintered into several factions and has a history of bizarre acts of terrorism.

Palestine Liberation Organization leader Yasser Arafat denounced the hijacking and said his group had nothing to do with it.

The ship, seized yesterday off the Egyptian coast, attempted to go to a Syrian port today but was refused permission to dock. Late tonight it was reported south of Cyprus, where harbor lights were extinguished to prevent a landing.

Early Wednesday, Israeli radio, monitored by the British Broadcasting Corp., said the ship was anchored off Port Said, Egypt, and that negotiations with Egyptian officials were under way. The Israeli report was based on information provided by well-known radio ham operator Michael Gurdus who had intercepted messages from the Italian liner. The hijackers demanded to talk to the U.S., British and West German envoys in Cairo, the report said and added that Egyptian officials were trying to convince the pirates to start releasing some of the hostages.

In Washington, national security adviser McFarlane

Several nations condemned the hijacking, the most spectacular act of piracy in a quarter of a century, and governments most closely involved warily sought diplomatic solutions while reiterating their refusal to give in to the demands of terrorists.

Israeli officials promised cooperation with any government in efforts to end the hijacking but said they would not give in to the hijackers' demands. The Italian government, after a day of crisis meetings, said it would seek a peaceful solution, although there were precautionary movements by Italian warships and planes.

The Reagan administration reacted with caution, saying only that it had contacted other involved governments to discuss what "appropriate action" might be taken to end the drama. President Reagan said he thought it was "the most ridiculous thing," but made no detailed comment.

The Pentagon said U.S. warships were in striking distance of the hijacked vessel but had made no obvious moves to break their deployment patterns, probably for fear of inflaming the situation.

Radio broadcasts, most of them by the hijackers, provided the only direct messages from the pirated vessel yesterday, but it was impossible to determine whether the information was authentic or propaganda.

Early today there were broadcasts claiming that one American captive had been killed "and we will kill another." Later broadcasts indicated a second had been killed. Lebanese monitors said the broadcaster boasted, "We have a lot of them here. God is generous."

But late today, a man identifying himself as the ship's captain and speaking English with a heavy Italian accent, pleaded loudly into the microphone, "I have only one message: Please, please don't try anything on my ship," apparently referring to a rescue attempt. He added, "Everybody is very good on the ship. Everyone and everybody will be freed in a short time."

There was no indication whether the statement was made under duress and monitors noted that while identifying himself as the captain, the man did not use the name of Gerardo de Rosa, whom the ship's owners have identified as the captain.

Ship manifests indicate that 16 Americans were among the estimated 120 passengers on board the Achille Lauro, along with at least 300 crew members, when it was seized off the Egyptian coast Monday. Most of the 753 passengers who began the cruise last Thursday had left the ship in Alexandria for a visit to Cairo.

The earlier radio messages from the ship, monitored by several sources, indicated that the decisive moment for two unidentified American hostages may have come in midafternoon as the hijackers were demanding an international diplomatic team to discuss their demands.

The first reports came from the Christian militias' Voice of Lebanon radio, which claimed to have intercepted conversations between the hijackers and authorities in Tartus, a northern Syrian port.

The hijackers insisted that the ambassadors in Syria of France, Britain and West Germany act as intermediaries. When the pirates appeared to decide the response was too slow, one hijacker is said to have warned over the radio, "The delay is harmful for them" -- presumably a reference to the hostages.

Although diplomats in Beirut said that Italian and German diplomats were on their way to Tartus, at 2:42 p.m. local time the hijackers were said to have warned, "We have no more time. We will start executing at 3 p.m. sharp."

At 2:55, they warned, "We have five minutes only," and three minutes later they declared they were unwilling to wait longer.

The radio went dead. But 20 minutes later a transmission reportedly picked up by a ham radio operator said, "We will follow up with a second one."

Jesus Ferreiro of the Spanish maritime radio service Onda Pesquera told United Press International that in a ship-to-ship communication "we heard a crew member of a warship in the area say that the person executed was an American of about 40."

Western diplomats in Syria were quoted this evening as saying Italian charge d'affaires Pietro Cordone was informed by the Syrian Foreign Ministry of the hijackers' claims to have killed two Americans.

Cordone told reporters that Italy is "aligning itself with the American position, that is not to negotiate with terrorists."

The hijackers, who are led by a man calling himself Omar, have found themselves increasingly isolated on the high seas since seizing the ship. Syria, which backs one of the major PLF factions, denied the ship permission to enter its territorial waters.

Meanwhile, Arafat's mainline PLO, which is allied to one of the three factions of the PLF and opposed by the other two, suggested the hijacking was meant to undermine the PLO's efforts to pursue peace negotiations in the region.

PLO headquarters in Tunis said, "The PLO declares that we have no relationship to this operation or to any terrorist group, and we are trying our best to resolve it."

Salah Khalaf, the second-ranking PLO official, said the PLO is ready to do anything possible to help end the crisis. There were reports today that it was attempting to persuade the hijackers to return to waters near Egypt in order to find a settlement.

The Egyptian Foreign Ministry denounced the hijacking as a "tragic incident," and condemned "all acts of violence, whatever their source, that have multiplied in the region in the last few days" -- a statement that appeared also to condemn Israel's raid on the PLO headquarters in Tunis a week ago.

The list of Palestinian prisoners the hijackers want Israel to free reportedly is headed by Samir Qantari. Qantari was captured after a 1979 raid on the Israeli coast in which four Israelis died.

The number of hijackers aboard the Achille Lauro remains unknown, with most estimates ranging from seven to 12. Trudy Hill, a spokeswoman for San Sebastian radio in Spain, told The Associated Press that messages she had monitored indicated "there are 12 guerrillas on board, all of them young except for the leader of the group. They are very heavily armed."

Other reports said that in addition to weapons, the hijackers had large amounts of explosives.

Just how they seized control of the ship as it sailed from Alexandria to Port Said remained unclear.

Speculation among those who left the ship at Alexandria for a day trip to the pyramids suggested that some of the hijackers may have been on board since the ship left Genoa.

Passengers suggested that they could have boarded in Naples or Alexandria. But an Egyptian Embassy official in Washington said he had been told by Cairo that "nobody boarded the ship in Alexandria."

At Naples, "You could walk aboard the ship without a pass. There was no security whatsoever," said Al Strauss of Menlo Park, Calif., one of the passengers who left the ship in Alexandria.

The possibility of a rescue attempt by another boat on the high seas was considered unlikely by most of the foreign military and diplomatic sources interviewed here.

"The boarding of a liner under way is not a very easy thing to do," said one military attache, "even with the best-trained commandos."

The seizure of the ship appeared to be carefully calculated, cruise staffers suggested, since the passage from Alexandria to Port Said, where the passengers were to reboard the ship, is the period when the fewest passengers are aboard.