A mentally challenged Malaysian man was executed in Singapore on Wednesday, according to his family, after losing a lengthy court battle despite international condemnation and clemency petitions.

Nagaenthran K. Dharmalingam was arrested in 2009 for smuggling a minor amount of heroin into the city-state, which has some of the strictest drug prohibitions in the world, and was sentenced to death the following year.

Because of worries about his intellectual disability, the intention to hang him drew worldwide condemnation.

"It is unbelievable that Singapore proceeded with the execution despite international appeals to spare his life," his sister Sarmila Dharmalingam told AFP.

The 34-year-old was executed in the early hours of the morning, she said from Malaysia, leaving her family "very heartbroken" and "shocked".

Nagaenthran spent more than a decade filing legal challenges, but they were all rejected by Singapore's courts, and the city-president state's rejected clemency requests.

His body will be sent to the Malaysian city of Ipoh for burial, according to his sister. Requests for comment from Singaporean officials went unanswered.

Singapore dismisses death sentence appeal

Earlier, Singapore's highest court denied a mentally challenged Malaysian man's last-ditch appeal against a death sentence, leaving his family "devastated" and "shocked" by the decision.

Nagaenthran K. Dharmalingam, 34, was caught in 2009 for bringing a minor amount of heroin-one and a half ounces into the state of Singapore, which has some of the strictest drug prohibitions in the world.

His death sentence was planned for November, but his lawyers have maintained that imposing the death sentence on a mentally ill person violates international law.

The Court of Appeal, however, dismissed the case, stating that it had "no factual and legal basis," and that local legislation takes precedence over international law.