Leading human rights groups have called on Ukrainian authorities to launch an investigation into enforced disappearances after details emerged of a “secret prison” being used by the country’s security service.

The SBU, Ukrainian’s successor agency to the KGB, secretly held some prisoners for more than a year at a covert facility in Kharkiv, a city in eastern Ukraine, Amnesty International and Human Rights Watch said in a joint statement on Monday.

“The Security Service’s continued denial of enforced disappearances fosters a climate of lawlessness and perpetuates impunity for grave human rights violations,” said Tanya Lokshina, a senior Europe and Central Asia researcher at Human Rights Watch.

Thirteen people were released after a report into illegal detention and torture by both sides in the conflict was published in July, but five more are still being held in the secret compound, the statement said.

Ms Lokshina called the release “very good news,” but said there had not been official acknowledgement of the detention or release.

Enforced disappearances occur when police or security forces detain individuals and then try to conceal the whereabouts of fate of the prisoners.

The practice is considered a grave violation of human rights under international law, and has been used by both sides during the two-year war in eastern Ukraine.