Indonesia Thursday rejected appeals from the UN and EU to halt the execution of 14 drug convicts including foreigners, as speculation mounted they could face the firing squad in a matter of hours.

The group, including foreigners from Nigeria, Pakistan, India and Zimbabwe as well as Indonesians, have been placed in isolation on a prison island where Jakarta carries out executions.

Authorities stepped up preparations with ambulances carrying coffins seen crossing over to Nusakambangan island. Family members say they have been told the convicts will be executed on Thursday night, according to a lawyer and diplomat -- who were angry as it was earlier than they thought was allowed.

The attorney general's office, which oversees executions, could not be contacted for comment.

Indonesia last carried out executions in April 2015 when it put to death eight drug convicts, including two Australians, sparking international outrage.

But President Joko Widodo has defended the use of the death penalty to combat rising narcotics use.

UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein on Wednesday called on Indonesia to end the "unjust" use of the death penalty, while the European Union urged Jakarta to stop the "cruel and inhumane punishment, which fails to act as a deterrent".

Indonesian foreign ministry spokesman Arrmanatha Nasir defended the looming executions as "pure law enforcement".

"I need to emphasise that all the legal processes of the convicts have been completed, all their rights have been fulfilled -- we target the drug traffickers and not users," he said.

Jakarta faced accusations of breaking its own laws by apparently planning to hold the executions on Thursday.

Diplomats and lawyers say they were given the legally required three days notice of the plan on Tuesday afternoon and believed the earliest it could happen was Friday.

"I am very concerned -- this cannot happen," Ricky Gunawan, a lawyer for a Nigerian convict set to face the firing squad, told AFP.

"It is clearly against the law. This execution has been completely under secrecy from the start."

Syed Zahid Raza, the deputy Pakistani ambassador in Jakarta, said the family of a Pakistani man, Zulfiqar Ali, had been informed he would be executed Thursday night.

Spokesman Nasir said his ministry had fulfilled all their obligations by notifying foreign representatives.

Pakistan has been angered about the planned execution of Ali -- whom rights groups claim was beaten into confessing -- and summoned Indonesia's ambassador in Islamabad this week to convey their concerns.