Human Rights Watch has urged Bahrain to immediately free leaders of last year's uprising after an appeals court ordered a retrial, raising pressure for significant gestures to defuse resurgent street protests.

Bahraini authorities also heard a call from the UN human rights agency to move a jailed hunger striker to a civilian hospital. Opposition leaders have said the court's gesture was insufficient and street unrest would resume.

Majority Shi'ite Muslims, whose unrest is seen by the Sunni ruling elite as a subversive bid to put US-aligned Bahrain under the sway of Shi'ite Iran, complain of discrimination and marginalisation in political life.

The government says many Shi'ites hold state posts and help run the economy and that police and judicial reforms have begun. However, there has been no progress on the main opposition demand for a parliament with full powers to legislate.

The cassations court, the highest judicial body in the Gulf Arab state, shifted the case of 21 men who were convicted in a military court to a civilian court and freed one, lesser-known prisoner. Seven of the 21 are abroad or in hiding.

However, the court ruled the men would remain in jail, including Abdulhadi al Khawaja, who was being fed intravenously in a military hospital after nearly three months of hunger strike.

"More than a year after they were arrested, the Bahraini authorities have produced no evidence that the jailed leaders were doing anything but exercising their basic human rights," New York-based Human Rights Watch said in a statement.

"The Court of Cassation made no reference, however, to the fact that the defendants had merely been exercising their basic human rights."

Government officials were unavailable for comment.

"We have urged the Bahraini authorities to take steps to ensure the release of Mr al Khawaja and his transfer to a civilian hospital," UN human rights agency spokesman Rupert Colville said.

"There is no reason for him to be held incommunicado and he should be given immediate access to his family, to the Danish ambassador ... and to a doctor and a lawyer of his own choosing."

In the UK, the Foreign Office said the new trial should be completed quickly and expressed concern for Mr al Khawaja's health, calling for an "urgent and compassionate solution".

"We now urge the courts to move this forward urgently, with due process and transparency. We call for all other upheld convictions by the military courts to be reviewed without further delay," the Foreign Office said.