French President Francois Mitterrand joined Syrian President Hafez Assad today in seeking to deflect allegations that Syria was responsible for a string of anti-French terrorist actions.

Appearing at an unusual joint press conference in Damascus, the two leaders expressed hopes for an improvement in French-Syrian relations following a period of serious crisis. Mitterrand was making the first visit to Syria by a French head of state since the former French protectorate gained full independence in 1946.

Mitterrand flew back tonight to Paris, where he faces several other major overseas problems, including a Libyan challenge to France's credibility in black Africa and a wave of secessionist violence in the French Pacific island of New Caledonia.

French spokesmen refused to comment today on a claim by Libyan leader Muammar Qaddafi in an American television interview that all Libyan troops had finally been withdrawn from the former French colony of Chad. Earlier, U.S. and French sources said they believed that between 1,000 and 3,000 Libyan soldiers were still in the African state in violation of a troop withdrawal agreement with France.

Reports from Damascus suggested that Mitterrand's trip to Syria had failed to produce any significant breakthrough on the issues that divide France and Syria on the search for peace in the Middle East. The French president was reported to have rejected Assad's call for a European initiative for a United Nations-sponsored peace conference with both the United States and the Soviet Union participating.

At today's joint press conference in Damascus, Assad denied accusations that his country was behind a series of terrorist attacks against France in recent years. Newspaper reports here have linked Syria with the assassination of the French ambassador in a Syrian-controlled area of Beirut in 1981 and the Beirut truck bombing that killed 58 French soldiers in October 1983.

In the past, French officials have hinted that Syrian security services could also have been involved in a car bomb explosion at the Paris office of an anti-Syrian newspaper in April 1982 which killed one person and wounded 62. Following that incident, France's Socialist government expelled two Syrian diplomats here and temporarily recalled its ambassador in Damascus.

Insisting that Syria was against "any kind of terrorism," Assad attacked the western press for "making a big issue of the explosion of a single bomb" while forgetting the much greater "terrorism" practiced by Israel against civilians in Beirut.

Asked about Syria's possible role in the attacks on French buildings in Beirut, Mitterrand said: "There is nothing to prove that Syria was responsible. Since President Assad has always asserted that this was not the case, I do not see why his word should be doubted."

Mitterrand and Assad joined in calling for Israeli withdrawal from Lebanon. "South Lebanon must recover its liberty," Mitterrand said, while Assad pledged Syria's "full support, whatever sacrifices it takes, to Lebanon's sovereignty on all its territory."

The first part of Mitterrand's visit to Damascus was overshadowed by the dramatic return from exile of Assad's younger brother Rifaat, who headed the elite Defense Companies charged with keeping order in the Syrian capital. Reports by French correspondents accompanying the president suggested that the bitter rivalries that led to Rifaat's departure have not been resolved.

According to an official Syrian government announcement, all three Syrian vice presidents, including Rifaat and former foreign minister Abdul Halim Khaddam, were to have been at the airport for Mitterrand's arrival. None of them showed up.

That evening, Rifaat and Khaddam both attended a state banquet for Mitterrand. French journalists at the dinner said that each vice president arrived with a group of supporters and ostentatiously ignored the other throughout the proceedings.

In a dispatch from Damascus today, the Paris newspaper Le Monde reported that Rifaat's Defense Companies had been reduced from 50,000 men to under 18,000. The paper added, however, that President Assad has allowed his brother a personal guard of 3,000 to ensure his security.

Mitterrand's three-day stay in Damascus coincided with a visit by the chief of staff of the Libyan armed forces, Abu Bakr Younis Jabir. But there was no public indication that Syria had been involved as an intermediary in the dispute between France and Libya over Chad.

Political analysts here believe that Mitterrand will be obliged to take some step to shore up France's credibility with its allies in black Africa before Dec. 11, the date of the annual Franco-African summit to be held this year in Burundi. The French government is under pressure from moderate African countries to resist what they regard as a continuing threat from Libya.

The French investigative newspaper Canard Enchaine reported this week that French officials were drawing up contingency plans for a possible air raid against Libyan troops in northern Chad and their Chadian allies. French government leaders have said that they want to give Libya a chance to fulfill its promises before taking "necessary measures" to ensure compliance with the mutual troop withdrawal agreement reached last September.