Singapore hanged two drug traffickers on Friday, authorities said, bringing the number of prisoners executed in the last four months to 10, despite international calls for the city-state to abolish capital punishment.

The spate of hangings included the widely criticised execution of a man with limited cognitive function in April, and came after Singapore resumed executions in March after a hiatus of more than two years.

The prisons department said in a statement that Singaporeans Abdul Rahim Shapiee, 45, and Ong Seow Ping, 49, were executed.

Shapiee, a former driver for a ride-hailing service, was convicted of trafficking 39.87 grams of pure heroin, Singapore's Central Narcotics Bureau (CNB) said. An appeals court dismissed his last-gasp petition for a stay of the sentence.

Ong was Shapiee's co-accused in the same case. He was convicted of trafficking 51.69 grams of heroin, the CNB said.

Both were "accorded full due process under the law, and were represented by legal counsel throughout the legal proceedings", the CNB said.

Friday's hangings brought to 10 the number of prisoners executed since March 30, when a Singaporean man was sent to the gallows.

That hanging was followed by the execution in April of Malaysian citizen Nagaenthran K. Dharmalingam, which sparked international outrage due to concerns he had "borderline functioning intelligence and concurrent cognitive deficits", according to rights group Amnesty International.

Two Singaporean convicts were executed on July 7, and two more were hanged within days of each other later that month. Two men – a Singaporean and a Malaysian – were hanged on Tuesday.

Singapore has some of the world's toughest anti-narcotics laws and insists that the death penalty remains an effective deterrent against trafficking, despite pressure from international rights groups for use of the penalty to be limited or abolished.

The CNB said capital punishment is part of Singapore's "comprehensive harm prevention strategy".

The United Nations says that the death penalty has not proven to be an effective deterrent globally and is incompatible with international human rights law, which only permits capital punishment for the most serious crimes. (AFP)