A United Nations human rights watchdog has called on Bahrain to release three relatives of a prominent exiled activist, describing their detention as an unlawful act of reprisal over their family connection.

The UN's Working Group on Arbitrary Detention in a report on Monday said three people were being held because of their kinship to Sayed Ahmed Alwadaei, a human rights activist and government critic who now lives in the United Kingdom.

The group found that Bahrain arbitrarily detained his brother-in-law Sayed Nazar Alwadaei, cousin Mahmood Marzooq Mansoor and mother-in-law Hajar Mansoor Hassan without warrants and didn't give them the right to a fair trial.

The three were sentenced in October 2017 to three years in prison on the charge of planting a fake bomb.

The UN group said they were "deprived of their liberty, interrogated and prosecuted for their family ties with Sayed Ahmed Alwadaei and that these were acts of reprisals".

The panel, composed of five independent experts, also voiced concern at allegations of torture and mistreatment in custody.

"The Working Group considers that, taking into account all the circumstances of the case, the appropriate remedy would be to release Sayed Nazar Alwadaei, Mr Mansoor and Ms Hassan immediately and accord them an enforceable right to compensation and other reparations, in accordance with international law," it said.

The group said Bahrain denied the allegations, but offered no evidence to support its claims.

Crackdown on dissent

Alwadaei, director of the Bahrain Institute for Rights and Democracy, fled the country during the Arab Spring uprising in 2011, seeking "sanctuary in the UK".

This is the second time in less than a week that the UN has censured the kingdom after calling last Friday for the release of another prominent activist, Nabeel Rajab.

He has been sentenced to five years in jail for tweeting criticism of Saudi Arabia's air attacks in Yemen and accusing Bahrain's prison authorities of torture.

Reacting to the UN report, Alwadaei said in a statement: "It's difficult for me to live freely when I know that others are imprisoned because of my actions."

"I just hope that this important decision influences the Bahraini authorities to end the torment of my family and grant their release."

He called for "Bahrain's powerful allies in the West", including the United States, to apply the full weight of their authority behind the UN decision and cease their support.

Bahrain, where a Sunni-Muslim royal family rules over a Shia-majority population, has kept a tight lid on dissent since the Shia opposition staged a failed pro-democracy uprising in 2011.