Four French fighter jets will join NATO air patrols over the Baltics starting on Sunday, France's chief of defense staff said Wednesday during a visit to Washington.

General Pierre de Villiers said the four fighter aircraft, either Mirage 2000 or Rafale jets, would fly from a base in Poland, amid growing anxiety in Baltic countries over Russia's intervention in Ukraine.

Mali has denounced as a "cowardly and despicable murder" the death of a French hostage announced by one of the west African nation's top jihadist groups.

The Movement for Oneness and Jihad in West Africa (MUJAO), a splinter group of Al-Qaida's regional branch, told AFP on Tuesday Gilberto Rodrigues Leal was "dead, because France is our enemy" although it did not explicitly spell out that the Frenchman had been murdered.

They ended up there penniless after wandering from country to country for months.

Yahya, Aziz and 150 other Syrians swapped the brutality and death of a war zone for hand-to-mouth survival in a small park in a working-class suburb of Paris, squeezed in behind a hotel just a few metres away from a busy ring road.

Mock executions, hunger, thirst, cold, beatings, a makeshift chess game to pass the time... and a "surreal" snowball fight with their jailers.

Details are starting to trickle through of the ordeal experienced by the four French journalists who returned home Sunday after being held hostage for 10 months at the hands of the most radical of Syria's jihadist groups, the Islamic State of Iraq and the Levant.

France has "information" but no firm proof that Syrian President Bashar Assad's regime is still using chemical weapons, President Francois Hollande said Sunday.

"We have a few elements of information but I do not have the proof," Hollande said in a radio interview after he was asked about reports that Assad was currently using chemical weapons.

Four French journalists taken hostage in Syria were reunited with family and colleagues on Sunday in an emotional homecoming after 10 months in captivity during which they were chained together and held in dingy basements.

Looking thin and tired but overjoyed, the men hugged relatives and colleagues waiting at an air base southwest of Paris where they flew in early Sunday from Turkey.

Four French journalists taken hostage in Syria last year were freed on Saturday after a 10-month ordeal in the world's most dangerous country for the media.

French President Francois Hollande announced the release of Edouard Elias, Didier Francois, Nicolas Henin and Pierre Torres, saying they were "in good health despite the very challenging conditions of their captivity".

A close adviser to Francois Hollande resigned on Friday after allegations about an alleged conflict of interest and his extravagant lifestyle, in a fresh blow to the beleaguered French president.

In a statement to Agence France Presse, senior political adviser Aquilino Morelle, 51, said he was stepping down but denied any wrongdoing.

French forces have freed five Malian aid workers who were taken hostage in a February kidnapping claimed by one of the country's top jihadist groups, the presidents of France and Mali said Thursday.

"An operation by the French armed forces" freed the five workers -- four Red Cross employees and a veterinarian from another aid organization -- after "a terrorist group" kidnapped them on February 8 in Mali's restive north, the presidents said in a joint statement.

A French journalist who was sacked over a Tweet suggesting former first lady Valerie Trierweiler -- also a journalist -- slept her way into the role has launched an appeal against his dismissal.

Pierre Salviac, a former rugby expert for the radio network RTL, triggered a storm that cost him his job when he took to Twitter three days after Trierweiler's then-partner, Francois Hollande, was elected president in 2012.