UN human rights chief Navi Pillay on Thursday condemned death sentences imposed by military courts in Bahrain on protestors accused of killings as well as military trials for civilian activists.

"The application of the death penalty without due process and after a trial held in secrecy is illegal and absolutely unacceptable," the UN High Commissioner for Human Rights said.

"The defendants are entitled to fair trials before civil courts, in accordance with international legal standards and in keeping with Bahrain’s international human rights obligations," she added in a statement.

Bahraini authorities have referred 47 doctors and nurses to a military court after accusing them of abusing their posts to take part in anti-regime protests that were crushed in March.

The UN human rights office said four protestors were sentenced to death last week and three to life imprisonment for the alleged killing of two policemen. They were reportedly held without access to their families and limited access to lawyers, it added.

The Gulf kingdom, which is ruled by the Sunni Al-Khalifa dynasty, has come under strong criticism from international human rights organisations for its heavy-handed crackdown on Shiites, including medical staff.

Authorities have said that 24 people were killed during the unrest, most of them demonstrators.