An American mixed martial arts instructor here had unprotected sex with two 15-year-old girls, filmed the acts with his mobile phone and showed an obscene clip to a third girl who was only six years old.

When Joshua Robinson, now 39, was finally caught, he had in his possession 321 films containing child pornography with children as young as two years old.

This is believed to be the largest number of such films seized from a single person in Singapore.

Robinson, who used to be a senior visual effects artist, was jailed for four years yesterday after pleading guilty on Feb 13 to four counts of making obscene films and three counts of having consensual sex with minors.

He also admitted to one count each of being in possession of obscene films and exhibiting an obscene object to a child.

Twenty other charges were taken into consideration during sentencing, including four more counts of having sex with the minors and 12 counts of making obscene films.

The court heard that Robinson got to know the first teen, who was then 15 years old, on a dating website. They met on Sept 11, 2013, and went to his home at the Riverwalk Apartments in Upper Circular Road, where he filmed himself having unprotected sex with her on his mobile phone.

He had sex at least two more times with the girl after this, and he filmed both encounters.

He met his second victim, also 15 years old at the time, via another website in early 2015. Robinson told the girl he had a fetish for young girls in uniform, Deputy Public Prosecutor Nicholas Lai said in court.

The DPP added: "He told her, 'I have wanted to find (girls) as young as possible. I think 13 would be ultimate'."

When the pair met on June 23, 2015, he told her to be in her school uniform. They met at Clarke Quay MRT station before going to his home, where they had unprotected sex and he filmed the act without her consent.

When she left his apartment, the court heard she broke into tears, had a "mental breakdown" and told her parents, who alerted the police.

Two days later, officers seized his computer and portable hard drive, where they found 5,902 obscene films, of which 321 contained child pornography.

About a month later, while he was out on bail, he targeted a third girl.

On July 28, 2015, Robinson went to a martial arts gym near Little India where he was a "participant", and showed an obscene video to a six-year-old girl while her father was training.

DPP Lai said: "After the accused had shown (the girl) the video, he asked (her) whether she liked it. (The girl) did not reply.

"He then told (her) not to tell anyone by putting his finger to his lips and saying 'shhhhh'."

The girl told her father what had happened when they got home, and he alerted the police.

DPP Lai urged District Judge Crystal Ong to jail Robinson for at least four to five years, and fine him $20,000.

He added: "(The girls') respective ages were made known to the accused... even before they met.

"The accused was a prowler on the Internet who sexually groomed and morally corrupted his young victims."

Defence lawyer Mervyn Tan said his client is mentally stable with no paedophiliac tendencies. He told the judge that Robinson has realised he acted foolishly and has promised to change for the better.

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