ISLAMABAD, Pakistan (AP) _ A man with a Libyan passport being questioned in connection with the hijacking of a Pan Am jetliner has said he works for the Libyan government on foreign assignments, officials said today.

Federal Investigation Agency officials told The Associated Press that Salman Taraki was arrested after his name came up during the investigation into last Friday's hijacking of Pan Am Flight 73 in Karachi.

''We have a lot of suspicions regarding Taraki,'' said one FIA official.

FIA director general Obaidur Rehman told AP Thursday night that Taraki had a Libyan passport, but appeared to be a Palestinian. The passport appeared to be a forgery, he said.

Officials have identified as Palestinians the four gunmen who, dressed as airport security guards, seized the Pan Am Boeing 747.

Twenty people were killed and more than 100 injured when the four hijackers panicked and opened fired at the passengers and crew after holding the plane for nearly 17 hours.

Taraki was arrested in Islamabad at the airport Wednesday after arriving from Karachi. He has since been sent back to Karachi, according to FIA officials.

They said Taraki was accompanied by one or two companions who slipped away at the airport.

The FIA officials, who spoke on condition of anonymity, discounted local police reports Thursday that one of the men with Taraki had fled to the Islamabad diplomatic office of the Palestine Liberation Organization. But they could not explain why the police had made the statement.

The FIA officials said Taraki had told them he works for the Libyan government inspecting that country's diplomatic missions around the world.

Some of the officials said Taraki had been in Cyprus before coming to Pakistan about six weeks ago. Others officials said this had not been confirmed.

The hijackers had demanded that the Pan Am plane be flown to Cyprus, where they said they wanted to secure the release of three Palestinian prisoners. The cockpit crew of the plane escaped at the start of the hijacking.

The ordeal ended when the plane's lights failed and the gunmen opened fire on the nearly 400 passengers.

The Associated Press of Pakistan, the semi-official national news agency, reported Thursday that FIA officials had said Taraki was not the leader of the hijackers.

Friday is the Moslem sabbath and telephone calls to the Libyan Embassy went unanswered today.

An Urdu language newspaper, Hurriyet, reported in a brief dispatch that an attempt would be made to hijack a Pakistani airliner to force the release of the four Pan Am hijackers, who are being held in Karachi. The report, which gave no attribution, said a major alert had been ordered at all airports.

However, a check of all major airports found no signs of special security measures. Airport security officials said they had no information about a possible hijacking attempt and said no special alert had been ordered.