French serial killer Charles Sobhraj, responsible for multiple murders of young foreigners in the 1970s across Asia, was freed on Friday after spending almost 20 years in a Nepali jail.

Sobhraj, 78, whose life was chronicled in the successful series "The Serpent", was driven out of the prison in a blue police vehicle through a scrum of reporters.

The French national was taken to immigration detention where he was escorted inside wearing a medical face mask, brown woolly hat and blue puffer jacket ringed by police in bulletproof jackets.

Nepal's top court ruled on Wednesday that he should be deported within 15 days but his lawyer later suggested this might be delayed because of health issues.

"Once he is taken to the immigration, then it will be decided what would be the next course. He has a heart issue, so he wants to get treatment from the Gangalal hospital," lawyer Gopal Shiwakoti Chintan told reporters.

The court ordered Sobhraj, who had heart surgery in 2017, should be released on health grounds after serving more than three-quarters of his sentence for murdering a US tourist and a Canadian in Nepal in the 1970s.

A French foreign affairs ministry spokesman said on Thursday that its embassy in Nepal was monitoring the situation.

"If a request for expulsion is notified to them, France would be required to grant it since Mr Sobhraj is a French national," he said.

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 his first murder, a young American woman whose body was found on a beach wearing a bikini, in 1975.

Nicknamed the "bikini killer", he was eventually linked to more than 20 murders. (AFP)