Singapore on Wednesday hanged a prisoner convicted of conspiracy to smuggle one kilogram of cannabis, authorities said, ignoring international calls for the city-state to abolish capital punishment.

The execution took place despite a plea by the United Nations Human Rights Office for Singapore to "urgently reconsider" the hanging and calls by British tycoon Richard Branson to halt it.

"Singaporean Tangaraju Suppiah, 46, had his capital sentence carried out today at Changi Prison Complex," a spokesman for the Singapore Prisons Service told AFP.

Tangaraju was convicted in 2017 of "abetting by engaging in a conspiracy to traffic" 1,017.9 grams (35.9 ounces) of cannabis, twice the minimum volume required for a death sentence in Singapore.

He was sentenced to death in 2018 and the Court of Appeal upheld the decision.

Branson, a member of the Geneva-based Global Commission on Drug Policy, wrote Monday on his blog that Tangaraju was "not anywhere near" the drugs at the time of his arrest and that Singapore may be about to put an innocent man to death.

Singapore's Home Affairs Ministry responded Tuesday that Tangaraju's guilt had been proven beyond a reasonable doubt.

The ministry said two mobile phone numbers that prosecutors said belonged to him had been used to coordinate the delivery of the drugs.

In many parts of the world -- including neighbouring Thailand -- cannabis has been decriminalised, with authorities abandoning prison sentences, and rights groups have been heaping pressure on Singapore to abolish capital punishment.

The Asian financial hub has some of the world's toughest anti-narcotics laws and insists the death penalty remains an effective deterrent against trafficking.

But the UN's Office of the High Commissioner for Human Rights disagrees.

"The death penalty is still being used in a small number of countries, largely because of the myth that it deters crime," the OHCHR said in a statement Tuesday.

Tangaraju's family pleaded for clemency while also pushing for a retrial.

Wednesday's execution was the first in six months and 12th since last year in the city-state.

Singapore resumed executions in March 2022 after a hiatus of more than two years.

Among those hanged was Nagaenthran K. Dharmalingam, whose execution sparked a global outcry, including from the United Nations and Branson, because he was deemed to have a mental disability.

The United Nations says 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