Indonesia on Thursday rejected mounting international pressure to halt the looming execution of 14 drug convicts 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 transporting coffins crossing over to Nusakambangan island and cars heading for the penal colony in the evening carrying convicts' relatives, police and religious counsellors.

Family members say they have been told the convicts will be executed Thursday night, according to a lawyer and diplomat, a day earlier than had been expected.

President Joko Widodo believes Indonesia faces an emergency due to rising drugs consumption and has dramatically escalated the use of capital punishment, putting to death 14 drug convicts, mostly foreigners, since he took power in 2014.

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

UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein has 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".

But Indonesian foreign ministry spokesman Arrmanatha Nasir defended the upcoming executions as "pure law enforcement", adding "we target the drug traffickers and not users".

There have been concerns about legal irregularities in the cases of some facing imminent execution, with Amnesty International citing "systematic flaws" in several trials and noting a handful of clemency appeals were still pending.

These included Pakistani Zulfiqar Ali, whom rights groups say was beaten into confessing to the crime of heroin possession, leading to his 2005 death sentence.

Rights group the Justice Project Pakistan say that the 52-year-old father of six was "tortured relentlessly" during a three-day interrogation in a house by police, after which he required surgery.

An internal investigation by the Indonesian government also found numerous problems with his case.

- Forced confessions -
Indonesian woman Merri Utami, who was caught with heroin in her bag as she came through Jakarta airport, claims she was duped into becoming a drug mule, and that police forced her into confessing with beatings and sexual harassment.

Rights groups have mounted a campaign to save her, and 10 women's rights activists were detained by police Thursday in Cilacap -- the port city closest to the prison island -- as they rallied in support of her.

Family members of Michael Titus Igweh, a Nigerian prisoner, also protested his looming execution, saying his case was still under review.

"I don't think this is fair. They should fulfil his legal rights first," Igweh's sister-in-law Nila, who gave just one name, told reporters in the port city.

Pakistan and India have been making last-ditch diplomatic efforts to save their convicts, with Islamabad this week summoning the Indonesian ambassador to convey their concerns.

Support for the death penalty in Indonesia is generally high but there has been some public opposition.

Dozens of protesters held a vigil late Thursday in front of the presidential palace in Jakarta, arranging candles in the shape of a noose and laying posters emblazoned with a bloody hand and the words "stop the executions" on the ground.

Diplomats and lawyers were angered after authorities notified family members the executions would take place Thursday, saying they believed they could only be held Friday after the end of a legally required, 72-hour notice period.

Authorities have not released a detailed list of the convicts who will face the firing squad but the Jakarta-based Community Legal Aid Institute said the group consists of four Indonesians, six Nigerians and one each from Pakistan, India, Zimbabwe and South Africa.

Indonesia had an unofficial moratorium on the death penalty between 2008 and 2012 but resumed executions in 2013 under Widodo's predecessor.