Singapore is expected to hang a Malaysian murder convict on Friday despite last-minute clemency pleas, his family and human rights groups said.

Kho Jabing, 31, was sentenced to death in 2010 for killing a Chinese construction worker in a robbery gone wrong and spent the next six years on a legal roller-coaster trying to avoid the gallows.

In a news conference late Tuesday, his sister Kho Jumai, 27, said the family was told in a letter from the Singapore Prisons Service that her brother would be executed on May 20.

Executions are normally carried out at Changi Prison before dawn on Fridays in Singapore.

The prison did not immediately respond to AFP's requests for confirmation of the execution date.

Only the Singapore president, on the advice of the cabinet, can grant clemency.

The president said last week that he will not grant clemency although the family is pleading for a last minute reprieve.

"I've done everything I can, I've sent letters all over the government, to anyone who would listen. Whether the letters were really received, I don't know because I don't have much education," said Kho's mother Lenduk Baling, speaking through an interpreter.

Malaysia also has capital punishment, executing murderers and drug traffickers by hanging, a system like in Singapore that dates back to British colonial rule.

Amnesty International Malaysia and Human Rights Watch have both released statements calling on Singapore to halt the execution and review the case.

After Kho was sentenced to death in 2010, Singapore amended its mandatory death penalty for murder, giving judges the discretion to impose life imprisonment under certain circumstances.

His case was reviewed and Kho was re-sentenced to a life term in 2013.

But after an appeal by prosecutors, Kho's death sentence was reinstated in January 2015.

An appeal was thrown out by a five-judge court last month, setting the stage for Friday's hanging.

Singapore executed four people in 2015, one for murder and three for drug offences, according to Singapore prison officials.

Rights groups have called on Singapore to abolish capital punishment but the government has rejected such calls, arguing death sentences are a deterrent to crime.