French citizen Benjamin Briere is still in an Iranian jail despite his recent acquittal by an appeals court, his lawyer has said.

Incarcerated in May 2020, Briere was sentenced to eight years in prison for espionage.

He is one of several foreigners who campaigners say Iran has jailed in a strategy of hostage-taking to extract concessions from the West.

His France-based lawyer, Philippe Valent, said in a statement sent to French news agency AFP that an Iranian appeals court had cleared his client of all charges and ordered his release on 15 February.

But Iran's Revolutionary Guards, the branch of the security forces entrusted with the preservation of the regime, "are keeping him imprisoned despite this declaration of innocence", he said.

"His release was prevented at the last moment," he said.

Held in the prison of Vakilabad in the eastern city of Mashhad, Briere is continuing a hunger strike which he started a month ago, and is "exhausted physically and mentally", Valent said.

Valent, who had previously described the espionage charges against his client as "fiction", said Thursday their "arbitrary nature" was "blatantly obvious".

He said Briere's family was demanding his immediate release.

"This situation is completely incomprehensible," Blandine Briere, said his sister.

She said the family had decided not to report the appeals court's verdict earlier in the hope that the situation might be resolved quietly.

Six French nationals are currently held in Iran.

The French foreign ministry has repeatedly condemned Iran for what it calls "hostage diplomacy".