The hijackers of a Kuwaiti jumbo jet with 53 people aboard killed one of their captives Saturday and threatened to kill more unless the plane was refueled, government officials said.

The hijackers later freed a sick passenger, the 58th person released from the 112 who were aboard when the plane was seized Tuesday. There are believed to be at least six hijackers.A senior Palestine Liberation Organization official met four times with the hijackers Saturday, raising hopes of a breakthrough in the deadlocked negotiations.

The Kuwait Airways Boeing 747 was hijacked on a flight from Bangkok to Kuwait and forced to land in Mashhad in northeastern Iran, where 57 people were released. It left Iran Friday and landed in Larnaca after being refused permission to land in Lebanon and Syria.

Cypriot officials identified the slain man as a security guard on the airliner.

But the Kuwait News Agency quoted the Persian Gulf state's information minister, Sheik Jaber Mubarak al-Sabah, as saying the victim was one of three military men returning from a vacation in Thailand.

The agency quoted unidentified officials as saying the released man is a 32-year-old Kuwaiti Shiite Moslem named Fadel Abdel-Rassoul Leeri. A Kuwaiti of that name was listed as a passenger.

Cypriot government spokesman Akis Fantis said the man's release "was a sign of goodwill by the hijackers."

Soon after his release, officials at the airport near Larnaca said the hijackers radioed the tower with an English statement reiterating their demand that Kuwait free 17 terrorists, all but one a Shiite Moslem, convicted there for a chain of bombings in 1983.

The remaining passengers are believed to be Arabs, including three members of Kuwait's extensive royal family.

Cypriot officials said negotiations between the hijackers and the Kuwaitis had stalemated Saturday, with "the hijackers demanding fuel so they can leave and the Kuwaitis insisting this must not happen."

But about three hours after the killing, the PLO's deputy director in Cyprus, Malaz Abdo, and two Cypriot officials were driven to the plane, parked at the eastern end of the beachside runway. They held a series of talks with the hijackers, but no details of the negotiations were released.

Cyprus Television reported that the hijackers first proposed releasing five passengers in exchange for fuel and the aircraft's departure, then proposed releasing half the passengers for about half the 17 terrorists held in Kuwait.

It said the hijackers later offered to release half the passengers in exchange for the three terrorists facing death sentences in Kuwait, with the remaining passengers to be freed when the release of the terrorists was confirmed. Cypriot authorities, at the insistence of Kuwaiti officials, rejected the proposals, according to the television report.

In London, the Sunday Telegraph quoted military sources as saying a 35-member special British military unit was flown to Cyprus Friday night to support a possible Cypriot-led rescue mission. London's Sunday Express reported Britain flew in special surveillance equipment to monitor activity aboard the jet.

Britian's Foreign Office only would say it was in touch with authorities in Cyprus, and Cypriot officials stressed that the government has not requested British military assistance.

At a news conference in Moscow, PLO chief Yasser Arafat said he would try to help resolve the crisis at the request of Kuwait. But he added: "The policy of the PLO is completely against these kinds of events. No Palestinian has been involved in this."

Saturday's slaying, the first in the five-day ordeal, came two minutes after a deadline set by the hijackers for the Cypriots to provide fuel.

The man's body, his hands tied, was tossed out of the airplane door onto the tarmac. An ambulance drove to the plane, medics put the body on a stretcher, and it drove away. A government spokesman said the man was shot three times in the head and had a broken neck. It was not clear if the neck injury resulted from the fall.