DUBAI, United Arab Emirates -- A court in Bahrain upheld Monday a five-year prison sentence against prominent activist Nabeel Rajab over tweets alleging torture at one of the country's prisons and criticism of the Saudi-led war in Yemen.

Rajab's son Adam Rajab and four human rights groups confirmed the ruling. Rights groups criticized it as a miscarriage of justice.

Bahrain's Court of Cassation, its top judicial body, upheld the conviction on charges of "spreading false rumors in time of war," ''insulting public authorities" and "insulting a foreign country," according to Amnesty International. The rights group described Nabeel Rajab as a "prisoner of conscience" who has already spent two years behind bars, including nine months in solitary confinement.

The Bahrain Institute for Rights and Democracy said Rajab's sentencing was over re-tweets of critical comments made by others about alleged torture at prisons in Bahrain, and tweets he wrote condemning airstrikes in Yemen mounted by the Saudi-led coalition, of which Bahrain is a member.

Rajab is also serving a two-year sentence over television interviews he gave that included criticisms of Bahrain, a small island-nation off the Saudi coast that's home to the U.S. Navy's 5th Fleet.

He's been detained since June 2016 as part of a wide crackdown on dissent in Bahrain.

A major figure in Bahrain's 2011 protests that saw tens of thousands from the country's majority Shiites demand greater rights from the Sunni-led monarchy, he is also the co-founder and president of the Bahrain Centre for Human Rights and a founding director of the Gulf Center for Human Rights.

In the years since the 2011 protests, Bahrain has dismantled opposition groups, imprisoned activists and revoked citizenship of over 700 people.

Amid the crackdown, local Shiite militant groups have carried out attacks on security forces.