UNITED NATIONS (AP) _ A Romanian U.N. official left his Geneva post last December and never returned from a trip to Bucharest despite repeated requests by Secretary- General Javier Perez de Cuellar that he do so, a spokesman reported Thursday.

Francois Giuliani, the secretary-general's spokesman, told reporters, ''We are not clear'' whether the official's subseqent resignation was voluntary or submitted under duress.

''The secretary-general remains in touch with Romania on this matter,'' Giuliani said at a press briefing.

The official, Liviu Bota, had been on loan to the United Nations by the Romanian government for the past 13 years. Since 1980, he had been director of the Geneva-based U.N. Institute for Disarmament Research.

Last December, Giuliani said, Bota left Geneva and traveled to Bucharest. He gave no notice of his intention to remain in his home country. Beginning in mid-January, the U.N. chief made repeated requests for Bota to return to duty in Geneva and also to attend meetings in New York.

Bota was ''unable to do so,'' Giuliani said, adding that Perez de Cuellar then took up the matter with Romania's U.N. mission and with ''Romanian authorities at the highest levels.''

Perez de Cuellar designated a special representative to go to Romania and pursue the matter in Bucharest, but the latter's visit ''could not take place,'' Giuliani said without elaboration.

On March 12, Romania's U.N. ambassador, Teodor Marinescum, notified Perez de Cuellar that Bota had resigned from his U.N. post. The secretary-genera l then sent a cable to the Romanian Foreign Ministry requesting that Bota submit his resignation in person.

Instead, a letter of resignation was received in New York on March 18.

On Wednesday, the Romanian envoy informed Perez de Cuellar that Bota is now a director in the Foreign Ministry in charge of coordinating general international questions.

There have been past cases in which U.N. officials have been detained during visits to their East Bloc countries and the United Nations has tried to intervene without results.

One of the most publicized cases involved Alicja Wesolowska, a U.N. secretary working in New York, who was arrested while vacationing in her native Poland and given a seven-year sentence in August 1979 for allegedly spying for an unspecified NATO power.

An East German official of the Paris-based U.N. Educational, Scientific and Cultural Organization - Percy Stultz - was arrested in March 1980 while visiting his country and convicted of having engaged in anti-state activities.

Both have since been released but have not been allowed to leave their countries to resume their U.N. jobs.