Ban Ki-moon, the UN secretary-general, has appealed to Indonesia to stop the planned executions of nine prisoners within days for drug-related crimes.

The nine - from Australia, Nigeria, Brazil, Ghana, the Philippines, and Indonesia - have been transported to the high-security prison island of Nusakambangan, where they are set to face a firing squad, despite mounting international criticism.

Indonesia has so far informed eight of the death row inmates, including two Australian members of the so-called "Bali Nine" group, and one Nigerian, that they will be executed in a matter of days, possibly as soon as Tuesday.

Officials said earlier that a 10th prisoner, the Frenchman Serge Atlaoui, who was expected to be among the group being put to death, would not be included in the forthcoming batch as he still had an outstanding legal appeal.

The United Nations opposes the death penalty in all circumstances and in a statement, Ban's spokesman said the secretary-general urged Indonesian President Joko Widodo to "urgently consider declaring a moratorium on capital punishment in Indonesia, with a view toward abolition".

'Most serious crimes'

Indonesia has harsh punishments for drug crimes and resumed executions in 2013 after a five-year gap. Six executions have been carried out so far this year.

"Under international law, if the death penalty is to be used at all, it should only be imposed for the most serious crimes, namely those involving intentional killing and only with appropriate safeguards," Ban's spokesman said in a statement.

"Drug-related offences generally are not considered to fall under the category of 'most serious crimes'," he said.

The foreign drug convicts have all lost appeals for clemency from Widodo, who argues that Indonesia is fighting a drugs emergency.

The Australian government, which has campaigned against the death penalty, said it had been informed that the execution of its citizens, Myuran Sukumaran and Andrew Chan, who were reportedly leaders of the drug trafficking ring, was "imminent".

"Nothing can be gained and much will be lost if these two young Australians are executed," said Julie Bishop, the Australian foreign minister.

"I again respectfully call on the president of Indonesia to reconsider his refusal to grant clemency. It is not too late for a change of heart."