France's President Emmanuel Macron is urging Algeria to free an 80-year-old writer who was sentenced on Tuesday to five years in prison, following accusations he had undermined Algeria's territorial integrity.

Boualem Sansal was arrested in Algeria last year after apparently telling a French far-right media group that, during the colonial era, France gave too much land to Algeria and too little to Morocco.

During his detention the French-Algerian author has spent time in hospital for ill-health.

His case has sparked a wave of support from intellectuals and politicians, including Nigerian Nobel Prize-winning author Wole Soyinka, Indian-British novelist Salman Rushdie and French officials.

One conservative mayor of a Parisian suburb is promising to hand out free copies of Sansal's books to every 18-year-old in his district, and says the author's sentencing in Algeria is an attack on free speech.

The French president meanwhile told a news conference: "I hope there can be humanitarian decisions by the highest Algerian authorities to give him back his freedom and allow him to be treated for the disease he is fighting."

Back in February, Macron sent a stark warning about the writer's plight, saying "Boualem Sansal's arbitrary detention, on top of his worrying health situation, is one of the elements that need to be settled before confidence [between our countries] can be fully restored."

Boualem Sansal now finds himself at centre of a deepening diplomatic row.

"He has unwillingly become a pawn in the troubled relationship between Paris and Algiers," a committee of his supporters in France said recently.

Algeria was once a prized French colony and fought a dogged war of independence eventually winning its sovereignty in 1962.

Relations have long been strained between the two countries but reached a new low last year, when France backed Morocco's claim to the disputed territory of Western Sahara, where Algeria backs the Polisario group fighting for the territory's independence.

Algiers responded to that slight by withdrawing its ambassador to Paris.

Three years earlier, Algeria severed diplomatic ties with Morocco.

Following Wednesday's court ruling, Sansal's lawyer pleaded to Algeria's President Abdelmadjid Tebboune to show "humanity" to the writer.

Sansal is well known for his anti-Islamist views and is an outspoken critic of the Algerian government.

His detractors say he is a darling of the far-right who appeases their prejudices.

Far-right French leader Marine Le Pen has called Sansal a "fighter for liberty and a courageous opponent of Islamism".

His age has previously been reported as 75, but his publishers Gallimard say he is in fact 80.

Sansal's best-known works include 2084 - a satire about religious radicalism which won the French Academy's Grand Prix of the Francophonie a decade ago.

His next novel, Vivre, is to be published in May and tells the story of a select group of people who are chosen to colonise a new planet as Earth nears apocalypse.