French serial killer Charles Sobhraj, responsible for multiple murders in the 1970s across Asia, was due to arrive in France Saturday after almost 20 years in prison in Nepal.

Nepal's top court ruled on Wednesday that he should be freed on health grounds and deported to France within 15 days.

On Friday, he was released and put on a flight at Kathmandu airport to take him to Paris via Doha. While on the flight to Doha, he insisted to an AFP journalist that he was "innocent".

"I feel great... I have a lot to do. I have to sue a lot of people. Including the state of Nepal," Sobhraj told AFP on Friday onboard the plane.

Asked if he thought he had been wrongly described as a serial killer, the 78-year-old said: "Yes, yes."

He is due to land in the French capital on Saturday morning.

- 'Bikini killer' -

Sobhraj's life was chronicled in the series "The Serpent" co-produced by Netflix and the BBC.

Born in Saigon to an Indian father and a Vietnamese mother who later married a Frenchman, Sobhraj embarked on an international life of crime and ended up in Thailand in 1975.

Posing as a gem trader, he would befriend his victims, many of them Western backpackers on the 1970s hippie trail, before drugging, robbing and murdering them.

Suave and sophisticated, he was implicated in the murder of a young American woman whose body was found on a beach wearing a bikini in 1975.

Nicknamed the "bikini killer", Sobhraj was eventually linked to more than 20 murders.

He was arrested in India in 1976 and ultimately spent 21 years in jail there, with a brief break in 1986 when he drugged prison guards and escaped. He was recaptured in the Indian coastal state of Goa.

Released in 1997, Sobhraj lived in Paris, giving paid interviews to journalists, but went back to Nepal in 2003.

- 'Karma' -

He was spotted in a casino playing baccarat by journalist Joseph Nathan, one of the founders of the Himalayan Times newspaper, and arrested.

"He looked harmless... It was sheer luck that I recognised him," Nathan told AFP on Thursday. "I think it was karma."

A court in Nepal handed Sobhraj a life sentence the following year for killing US tourist Connie Jo Bronzich in 1975. A decade later, he was also found guilty of killing Bronzich's Canadian companion.

Talking to AFP among bemused fellow Qatar Airways passengers on Friday, Sobhraj insisted he was innocent of the killings in Nepal.

"The courts in Nepal, from (the) district court to high court to supreme court, all the judges, they were biased against Charles Sobhraj," he said.

"I am innocent in those cases, OK? So I don't have to feel bad for that, or good. I am innocent. It was built on fake documents," he added.

Thai police officer Sompol Suthimai, whose work with Interpol was instrumental in securing the 1976 arrest, had pushed for Sobhraj to be extradited to Thailand and tried for murders there.

But on Thursday, Sompol told AFP he did not object to the release, as both he and the criminal he once pursued were now too old.

"I don't have any feelings towards him now that it's been so long," said Sompol, 90. "I think he has already paid for his actions."