A library assistant who had subjected two young sisters to anguish in separate episodes over the last six years was yesterday jailed for 18 months.

Tan Yao Min, 21, stalked the girls, now aged 18 and 14, and sent them letters with threats to rape and kill one of them. He admitted to three of six charges - criminal intimidation, intentionally causing alarm and unlawful stalking.

Tan, who has autistic spectrum disorder and suffers from immature personality, left two handwritten notes in a flier box at a unit next to a flat belonging to the victims' uncle on Jan 12 this year.

He stated, among other things, that he wanted to rape and impregnate the 18-year-old to make her "suffer''. "I like to kill her,'' he wrote. The second letter stated that he "lost everything'' and found life "meaningless''. He also asked them to report him to the police. He was arrested the next day after the older girl reported him.

While on police bail, he re-offended. On Feb 20, he penned two more letters saying that he loved the older girl and wanted to have sex with her. He hoped that he could have paid sex with the younger one, and a threesome in bed to prove he was a "man''.

He reiterated that life was "meaningless'' and expressed his wish to go to prison in the second note.

Tan had been stalking the girls between Jan 12 and Feb 24 this year, investigations showed. He waited for the younger girl at a bench near her home, followed her around the neighbourhood and sent her a Facebook invite.

In 2011, Tan had made statements that he wanted to have sex with the younger girl, then eight.

Tan, who was unrepresented, could have been jailed for seven years or more for criminal intimidation. The maximum punishment for stalking is a $5,000 fine and 12 months' jail; and for intentionally causing alarm, a $5,000 fine and six months' jail.