France revealed today that it has expelled two imprisoned Palestinians belonging to the extremist group Abu Nidal after secret negotiations with Syria.

French officials said that the two Palestinians were released from jail on Feb. 5 after serving half of the 15-year sentences that they received for the 1978 murder of a Palestine Liberation Organization representative in Paris. The released terrorists were subsequently expelled from France, official sources said.

The release of the two Abu Nidal members came on the last day of a series of mysterious bomb explosions in Paris, for which responsibility was asserted by a group demanding the release of Arab prisoners in France. French officials insisted, however, that there was no direct connection between the two events.

The release came also amid speculation over attempts to negotiate the release of four Frenchmen kidnaped in Lebanon last year and apparently being held by a Moslem extremist group there.

According to sources here, contacts between France and Syria on the subject of terrorism began in 1983, when the French government was seeking a truce with Abu Nidal. The group was suspected of involvement in a series of terrorist acts against French interests here and abroad in the early 1980s.

Prime Minister Laurent Fabius today defended the release of the two Jordanian-born Palestinians, Kayed Assad and Husni Hatem Abdul Kadir, calling it fully in accordance with French law. The penal code allows the government to free prisoners for good behavior after serving half their sentences.

"We have absolutely no desire to keep terrorists on French soil," the prime minister told a campaign meeting.

The release of the Palestinians came against a background of diplomatic contacts between France and Syria for the release of the four French hostages in Lebanon. In an interview last week, Syrian President Hafez Assad revealed that a tentative Franco-Syrian agreement for the release of the hostages collapsed in January because the kidnapers refused to honor it.

The Lebanese captors are reported to have demanded the release of a five-man team that killed a French policeman while trying to assassinate former Iranian prime minister Shahpour Bakhtiar in July 1980. This demand posed greater political and legal problems for the French government than the case involving Kayed and Hatem.

In his interview with the Paris daily Liberation, Syria's Assad praised French President Francois Mitterrand for "doing the best he could within the framework of French laws." The remark was seen here as significant because it implied that Syria was satisfied with an arrangement short of the release of Bakhtiar's would-be assassins.

Indicating that he was angry with the Lebanese extremists for failing to honor his agreement with Mitterrand, Assad said that he would be prepared to consider military action to free the French hostages if he knew where they were held.

"Nothing in my eyes justifies the fact that an agreement concluded between the French president and myself should be put in doubt," Assad said.