A Bahraini court has ordered a retrial for 21 men convicted in a military court of leading last year's pro-democracy uprising, but said they would remain in jail until new verdicts are reached, the official news agency said on Monday.

Among those facing a new trial in a civilian court is Abdulhadi al-Khawaja who has been on a hunger strike for over two months and was at risk of dying.

"The court is (ordering) that the trial take place again and that testimony from prosecution and defence witnesses be heard once more as if it is a new trial," BNA said.

"Cassation Court rulings do not allow for releasing defendants as long as they were imprisoned when presented to the first trial," it added.

Defence lawyer Mohammed al-Jishi, who attended Monday's session, said the judge stated that the men would not be released. International rights groups have said they should be freed without condition.

The convicted men - none of whom appeared in court - are believed to be among hundreds that an international rights commission said in November were tortured during a period of martial law.

They were sentenced by a military court last year for organising the protests led by majority Shi'ites that threatened the Sunni monarchy's grip on power.

The main charge was "forming a terrorist group with intent to overturn the system of government", but also included collaborating with a foreign state - an apparent reference to Shi'ite power Iran.

Eight of the group were given life sentences, including Khawaja and opposition leaders Hassan Mushaimaa and Abdulwahhab Hussein.

They had expressed support for turning the Gulf Arab state, which is host to the U.S. Fifth Fleet, into a republic.