Singapore  - A teenage boy who killed his lover's husband was sentenced to indefinite detention until Singapore's president agrees to his release, news reports said Wednesday.

Muhammad Nasir Abdul Aziz repeatedly stabbed Manap Sarlip, 29, in May 2007 outside the couple's flat.

The wife, Aniza Essa, 25, was sentenced last week to nine years in prison for instigating the teenager.

Murder carries a mandatory penalty of death by hanging in the city-state for those above the age of 18. For younger defendants, cases are reviewed every four years.

Subhas Anandan, the youth's lawyer, on Tuesday described Aniza as a "manipulative monster" for playing mind games on the boy to coerce him into killing her husband, The Straits Times said.

Anandan said that Aniza cleverly exploited the teenager's feelings with promises of love if he pulled off the killing and threats to end the relationship if he failed.

The lawyer produced a letter written by the teenager. "I'm so sorry for being so stupid to listen to her and murder an innocent man," it said.

Justice Kah Ting Chiu said the evidence showed that Muhammad Nasir was not the initiator of the crime and had been "egged on by somebody" he thought he loved. (dpa)s