A Swiss man will go on trial on allegations he sexually abused more than 80 boys - some as young as nine years old - in Thailand, authorities in Switzerland said on Friday.

The unnamed man has admitted to taking thousands of pornographic pictures of the boys, but denied allegations that he molested some of them and pushed some into prostitution, according to prosecutors in the western Swiss canton of Fribourg.

The abuse is alleged to have taken place since 2001.

The man had previously been convicted of sexually abusing children in Switzerland: in Fribourg in 1980 and in the southern canton of Valais in 1991.

He had moved to Thailand that same year and only returned to Switzerland in 2014.

Victims as young as nine

The man will go before the Gruyere criminal court later in March on charges of human trafficking, encouraging prostitution, sexual acts with children and child pornography, prosecutors said.

Most of the victims were young adolescents, but some were as young as nine years old, they said, adding that a number of the boys had been abused over the course of several years.

The suspected abuse happened in Pattaya in southern Thailand, where the man ran a bar frequented by European nationals, and in Nong Hoi in the north, where he lived, according to prosecutors.

Discovering that he was under investigation in Thailand over paedophilia-linked charges, authorities in Fribourg opened their own probe in early 2015 and quickly detained the man, who had been planning to head back to Asia.

Investigators travelled to France and Thailand during the course of the investigation, and also stumbled on elements casting suspicion on a German citizen that they passed on to prosecutors in Cologne, Germany, the statement said.

In January, a 70-year-old British paedophile, Mark Frost, was jailed for life after admitting to 45 sex offences against children in the UK and Asia, including at least nine children in Thailand from 2008 to 2012.

ECPAT, the anti-child exploitation, pornography and trafficking organisation, said in a 2015 report: "While Thailand's laws are generally strong when it comes to protection for children against sexual exploitation, there are still gaps that remain in regard to legislation that affects the prosecution of travelling child sex offenders."