The sister of a Frenchman being held on suspicion of espionage in Iran told French media on Tuesday her brother was a tourist and the case against him was baseless.

Benjamin Briere was arrested in May 2020, allegedly while flying a drone.

He is facing charges of espionage and "propaganda against the system", according to a lawyer representing another French national held in Iran.

Briere's sister, Blandine Briere, said he was a tourist on a road trip that began in 2018 and took him first to Scandinavia and then to Iran.

"His dream was to travel with his campervan, meet new people and discover new landscapes," she told France Info radio.

"The accusations are baseless. He was a tourist who was hoping to snap pretty pictures. And here we are, in the middle of an outrageous story."

Briere said her brother called her regularly during his travels, but in May 2020 the calls stopped.

"Suddenly there was no more news," she said.

She contacted the French embassy in Iran and was eventually told that Benjamin had been arrested.

Commenting on Iranian claims that her brother was taking photographs in a prohibited area using a drone when he was arrested, Briere told Le Point magazine: "We're talking about a simple gadget, a drone you can get for 100 euros ($120) in any shop, meant for tourism photos, that's all."

Briere is being held in Vakilabad prison in Mashhad, Iran's second largest city, in a cell with a dozen other prisoners, according to Le Point.

"Benjamin is holding up physically, even if it's very tough," his sister said, adding she had spoken to him twice on the telephone over the past two months.

On Monday, a spokesman for the French foreign ministry said Briere was "benefitting from consular protection" and that its embassy in Tehran was in "regular contact" with him.

"We get regular updates from Benjamin, but 10 months after his arrest that's no longer enough," Blandine Briere said. "We need to be sure that France will do everything to get him out of there."

Arrests of foreigners in Iran -- especially dual nationals, who are often accused of espionage -- have multiplied since former US president Donald Trump in 2018 unilaterally withdrew the United States from a nuclear deal with Iran and reimposed harsh sanctions against Tehran. 

Iran has conducted several exchanges of foreign prisoners, including researchers, with countries holding Iranian nationals.