Singapore authorities defended late on Wednesday their decision to execute a Malaysian man convicted of drug smuggling, in response to international blacklash over their use of the death penalty despite pleas of mental disability.

Nagaenthran Dharmalingam, 34, had been convicted of smuggling at least 42 grams (1.48 oz) of heroin into Singapore, which has among the world's harshest narcotics laws. He was hanged on Wednesday morning, after multiple legal challenges and appeals for clemency on the grounds that he had an intellectual disability, failed.

His lawyers and activists have said Nagaenthran's IQ was found to be 69, a level recognised as an intellectual disability.

But Singapore's Central Narcotics Bureau said in a statement his actions were "a deliberate, purposeful and calculated decision," and reiterated court findings that "he knew what he was doing."

The Attorney General's Chambers said in a separate statement Nagaenthran was given a fair trial and had "exhausted his rights of appeal and almost every other recourse under the law over some 11 years."

The case attracted international attention, with a group of United Nations experts and British billionaire Richard Branson joining human rights activists to urge Singapore to commute his death sentence.

The European Union and Amnesty International were also among several voices that called the punishment "inhumane" and urged Singapore to impose a moratorium on executions.

The city-state's government says the death penalty is a deterrent against drug trafficking and most of its citizens support capital punishment.

Another Malaysian drug trafficker, Datchinamurthy Kataiah, is due to be executed on Friday.