Image copyright YouTube Image caption US Mixed Martial Arts instructor Joshua Robinson was jailed four years on child sexual assault charges, a sentence being criticised by Singaporeans for being too light

There is public anger in Singapore over what is seen as a too lenient sentence for a US child sex offender.

US Mixed Martial Arts instructor Joshua Robinson, 39, was jailed for four years last week for having sex with two teenage girls and filming the acts.

Officers also seized thousands of illegal pornographic films from his flat, including 321 featuring children, the largest haul ever in Singapore.

A petition was launched calling the sentence "absolutely intolerable".

For each charge of having sex with a minor, Robinson could have been jailed in Singapore for up to 10 years and fined.

But on Wednesday, Singapore's Public Prosecutor said it would not be appealing for a harsher sentence. It said the two 15-year-old victims had "consented to the sexual acts", meaning the sentence was appropriate.

Videos uncovered at home

Specialising in the art of Brazilian Jiu Jitsu, Robinson had been living in Singapore for about six years.

He was well-known in the MMA community, and had previously been chosen to appear in a state campaign for Contact Singapore, a local employment body promoting expat talent.

The court heard that he had met one of the teenagers on a dating app in early 2015. After months of exchanging messages, they met up in person and he brought her back to his apartment where he filmed them having intercourse.

Unaware of the video, the girl then informed her parents about the assault and they lodged a police report.

Robinson was arrested in June 2015. A police raid uncovered thousands of obscene movies as well as a folder of saved films, including one of him engaging in sexual intercourse with another 15-year-old girl he had met two years before.

Image copyright Getty Images Image caption Singapore is known for its tough approach to law and order

He later pleaded guilty to nine charges; they included three counts of having sex with a minor under sixteen and one for showing an obscene film to a six-year-old girl, which he did while out on bail.

He was given a total of four years in prison.

Outrage in Singapore

Singapore's crime rate is among the lowest in the world, and its judicial system one of the toughest. Lengthy jail terms and canings are common.

So public anger erupted in the media and online, as Robinson was seen as having got off lightly. Some asked whether a foreigner had been allowed to escape a harsher punishment.

More than 27,000 people have signed a petition calling for a revision of the sentencing.

"This is totally absurd," said Singaporean Michael Liew Khong in a post comment on state broadcaster Channel NewsAsia's Facebook page. "A four year sentence for a paedophile? What kind of laws do we have here? It's never about the nationality but the seriousness of the crime."

Other Facebook users like Chiaming Cho agreed. "This isn't an issue about locals versus foreigners. This is about a tough enough sentence to deter would-be offenders for a crime that may have lasting effects of a lifetime."

Edwin Yeo highlighted the amount of videos featuring children found in Robinson's possession. "This is taken more seriously in other nations. Why isn't our justice system looking at that? Robinson will probably be out in two years for good behaviour. This is infuriating."

The lack of corporal punishment was also called into question by some netizens.

"I do not understand why there isn't any caning being meted out in the sentencing," said Mag Kan. "Caning has always been part of sexual assault sentencing. Why is he being exempted from that?"

In its statement on Wednesday, the Attorney-General's Chambers (AGC) acknowledged public unhappiness but said the four-year sentence was "broadly in line with relevant sentencing precedents" and that by pleading guilty, the victims had been spared having to testify.

It stressed Robinson had been convicted of neither sexual assault nor statutory rape, as the girls were above the age of 14 and had consented. "Caning is not provided for any of the offences Robinson was charged with," it said.

The AGC said it did "not differentiate between Singaporeans and non-Singaporeans", but that it would be discussing with government "whether the relevant legislation should be reviewed to enhance sentencing for some of the offences".