Two drug traffickers were hanged in Singapore on Thursday, bringing to four the number of executions in the city-state since March, the prison service said.

The two convicts, a 48-year-old Singaporean and a 31-year-old Malaysian, were convicted of heroin trafficking in the same case in 2016.

The Malaysian convict had attempted a last chance appeal on Wednesday, but it was rejected by the Singapore justice.

After a two-year hiatus, Singaporean authorities resumed executions of death row inmates in March, with the hanging of a Singaporean drug trafficker. The following month, the execution of a mentally handicapped Malaysian, also convicted of heroin trafficking, raised an international outcry.

The application of the death penalty in Singapore constitutes "a flagrant violation of human rights", denounced Emerlynne Gil, an official of Amnesty International for Asia. She urged the Singaporean government to "end this shameful and inhumane punishment".

In a recent interview with the BBC, Singapore's Home Affairs and Justice Minister K. Shanmugam defended the use of the death penalty, saying there is "clear evidence that it is a serious deterrent to would-be drug traffickers”.