A former primary school teacher who has been in and out of jail over sexual crimes involving underage boys committed a string of offences after his last release from prison.

Chock Soon Seng, now 43, pleaded guilty in a district court yesterday to four counts of sexual penetration involving two of three underage boys he had met in 2018 and 2019.

Fourteen other charges involving the teenagers and the third victim will be considered during sentencing.

Chock was first sentenced to 10 months' jail in 2010 after he tried to persuade two 13-year-old boys to let him watch them perform a sexual act.

In 2014, the Singaporean was sentenced to three years' jail for offences including sexually abusing a 14-year-old boy and threatening to put his naked pictures online.

Chock, who is now in remand, was working as a freelance tutor at the time of his latest offences.

The court heard that his first victim, a 15-year-old boy, had in 2018 posted an advertisement on online classifieds website Locanto indicating that he was interested in looking for women.

Chock responded to it in April that year and the teenager told him that he was not a homosexual. But Chock persuaded him to "experiment" and said that it was "normal for straight men to perform sexual acts on one another".

In their court documents, deputy public prosecutors Chin Jincheng and Chong Yong said that Chock made no attempts to verify the teenager's age and instead, offered to pay him for sexual acts.

Chock continued to persuade the boy and he went to the teenager's home on April 23, 2018, when they performed oral sex on each other.

He later gave the boy an unspecified amount of cash and left.

The pair met again and performed sexual acts on each other at the boy's home on May 11, 2018.

Chock met the second victim, a 13-year-old boy, through gay dating app Grindr in 2018 and the teenager told him that he was a Primary 6 pupil.

The pair met on April 15 that year and they performed oral sex on each other at a staircase landing of a multi-storey carpark.

They met again two days later and they performed similar acts in a toilet cubicle at the Punggol Community Club.

Chock's offences came to light in June 2019, when the boy's mother found some sex-related conversations on her son's mobile phone.

She alerted the police and officers arrested Chock soon after.

Chock will be sentenced on April 9.

Chock's offences came to light in June 2019, when the boy's mother found some sex-related conversations on her son's mobile phone. She alerted the police and officers arrested Chock soon after.