Nine hijackers released their 28 hostages here today in exchange for a promise of safe conduct to Aden, capital of South Yemen, a Kuwait government spokesman said.

The Kuwait Information Ministry spokesman said three officials boarded the hijacked Kuwait Airlines Boeing 707 to serve as hostages pending approval of the agreement by the South Yemen government in Aden, about 1,200 miles south of here.

The three were the South Yemen ambassador to Kuwait, a Palestinian official, and the chief of Kuwaiti internal security, the spokesman said.

It was not immediately known if the British pilor and copilot were allowed off the jet or would remain aboard to fly the plane to Aden.

Earlier, airport sources said the hijacked plane, with the terrorists aboard, would leave Kuwait Sunday morning.

The Palestine Liberation Organization in a statement issued here identified the gang's leader as Abu Saed, 36. The statement said he was an accused thief, defrauder and extortionist who escaped Thursday from a guerrilla prison in Beirut.

Palestinian sources said Saed was convicted several times in secret trials by "revolutionary courts" of the Al Fatah guerilla organization.

Saed, who Kuwaiti officials said was "unbalanced," walked out of the plane and reportedly issued a bizarre demand that Israeli Prime Minister Menachem Begin should personally released Archbishop Hailarion Capucci, a Greek Catholic leader serving a 12-year prison sentence for smuggling arms to Palestinian guerrillas.

In Tel Aviv, an Israeli government official termed the demand for Capucci's release "preposterous."

In its statement, the PLO condemned the hijacking and disclaimed any connection with it.

The agreement was reached after long hours of negotiations at Kuwait airport in desert heat that reached 118 degrees. The final details were worked out between two hijackers and Kuwaiti Planning Minister Mohammed Youssef Adasani on the airport runway near the plane.

The hijackers had originally demanded that 300 prisoners held in Arab jails be freed in exchange for the lives of their captives. PLO officials here and in Beirut said the hijacking might have been aimed at Syria, which holds a number of Palestinian prisoners.

Kuwait's interior and defense minister, Sheikh Saad Abdalah Sabah, reportedly said the gunmen must first release the hostages and that they could then "take the plane and go to hell."

The plane was seized last night on a flight from Beirut to Kuwait. The hijackers demanded the pilot land at Kuwait to refuel. Since then, the jetliner has sat at the airport, ringed by Kuwaiti army commandos 400 yards away. The airport is 20 miles south of Kuwait city.

After hours in the stifling heat, the hijackers were "on the verge of collapse," a negotiator said. The gunmen finally allowed ground crews to hook up an airconditioning unit to the plane.

The hijackers threatened to blow up the jetliner if the commandos attacked. They reportedly were armed with grenades, at least one submachine gun and smaller weapons.

Kuwait Airlines officials said the jet was carrying 45 passengers and a crew of 10, including seven Britons, when it was hijacked. Airport sources here said all the passengers were Arabs.

At the defense minister's request, the gunmen released 18 hostages Friday night including three British crew members. The Kuwaiti ambassador to Lebanon and Kuwait's director of foreign investment were among those still aboard the plane.

The PLO statement identified Saed as a member of the radical Popular Front for the Liberation of Palestine-General Command. It said intelligence agents from an Arab country, which it did not name, helped him make his jailbreak Thursday.

Sources in Beirut said he had worked in Al Fatah, the guerilla group of PLO leader Yasser Arafat, and airport sources here said he claimed to belong to Al Fatah.

"We are not terrorists. We have a cause," he reportedly told negotiators.

Beirut sources close to Arafat said Saed's real name was Abdul Karim Abu Hamdi and that he was a heavy gambler who extorted money and took bribes when he was in charge of post office communications for the Palestinians in Lebanon during the 1975-76 civil war there.

It was the second time in the past month that hijackers boarded planes in Beirut and the third hijacking in the Persian Gulf region since the beginning of June.