DUBAI (Reuters) - The top court in Western-allied Bahrain upheld three-year jail terms against three relatives of a prominent political activist, a rights group said on Monday, in a case the United Nations says is an unlawful act of reprisal over family connections.

Home to the U.S. Navy’s Fifth Fleet and a British Naval Support Facility, the island state has prosecuted hundreds of protesters in mass trials and banned main opposition groups in recent years. Most of the leading opposition figures and human rights activists are imprisoned or have fled abroad.

Bahrain’s Court of Cassation rejected the appeal of three family members of exiled activist Sayed Ahmed Alwadaei, the London-based human rights group Bahrain Institute for Rights and Democracy (BIRD) said in a statement.

Bahrain’s public prosecutor, in a statement sent to Reuters, confirmed the sentences, saying the Court of Cassation had rejected the defendants’ appeal “in substance”. Monday’s ruling cannot be appealed.

Alwadaei, BIRD’s head of advocacy, said his relatives were convicted based on confessions gained by torture and that they were being “persecuted” for his activism.

Human Rights Watch, Amnesty International and other rights groups had urged Bahraini authorities on Sunday to drop the charges.

Alwadaei’s brother-in-law, Sayed Nazar Alwadaei, his cousin, Mahmood Marzooq Mansoor, and his mother-in-law, Hajar Mansoor Hassan, were sentenced in October 2017 on security-related charges. An appeal court upheld the sentence in December 2017.

Bahrain’s criminal investigation department said last month the three were sentenced for “terrorist crimes”, rejecting criticism by the U.N. Working Group on Arbitrary Detention that the three were held due to their kinship with Alwadaei.

The activist’s inlaws were each sentenced to three years for planting “fake bombs” while his cousin is serving an 11 year sentence under separate cases involving arson and explosives.

Bahrain has clamped down on dissent after pro-democracy protests in 2011, led mainly by Shi’ite Muslims in the Sunni-ruled country, were quashed with the help of fellow Gulf states.

Alwadaei, based in Britain, said authorities moved against his relatives after he took part in a London demonstration last year to protest against the attendance of Bahrain’s king at a royal horse show in Britain.

“I will not rest until my family is free. Their continued imprisonment is a shameful reminder of the UK’s weak position when dealing with human rights abuses committed by an ally country,” Alwadaei said in the statement.