The Ukrainian Security Service (SBU) has posted images from a Hollywood blockbuster film on Twitter in an apparent effort to mock serious allegations led by two respected international rights watchdogs of secret detentions and torture.

Three stills from the 2016 movie Suicide Squad -- in which jailed villains are coerced by a secret government agency into becoming covert antiheroes to save the world -- appeared in the September 1 tweet by the Security Service (@ServiceSsu) with a caption that read "Prisoners of SBU secret jail."

The post was subsequently deleted.

The message appeared to be aimed at bolstering the Ukrainian Security Service's denial of accusations by Amnesty International and Human Rights Watch (HRW) in a report in July titled "You Don't Exist" Arbitrary Detentions, Enforced Disappearances, And Torture In Eastern Ukraine.

Those groups also sent a letter to Ukrainian officials dated August 23 citing the cases of 12 men and one woman who they said were released in July and August from "abduction-style" custody in Kharkiv. More such individuals are believed to remain incommunicado in custody.

The report claims that, in addition to Kharkiv, there are at least three more such "informal detention sites" in the eastern Ukrainian cities of Kramatorsk, Izyum, and Mariupol.

'Pretrial Investigation Center'

Amnesty International and HRW interviewed two men allegedly kept in SBU captivity since December 2014. Both men said they were abducted from their homes, tortured, and forced to confess to being informants for pro-Russia separatists in eastern Ukraine.

The Ukrainian military has been fighting Russia-backed fighters since early 2014, when Russian troops occupied Crimea ahead of the peninsula's annexation and armed fighters led efforts to reject Kyiv's authority. Swaths of the Donetsk and Luhansk regions remain outside central government control.

Ukrainian authorities deny the existence of any secret jails.

The Kharkiv location said to house a secret site instead houses a "pretrial investigation center," SBU chief of staff Oleksandr Tkachuk said, according to the BBC's Ukrainian Service.

"The fact that many interrogations take place in this building means that many detainees have been in this building, and it is often accused of being a secret jail of the SBU," Tkachuk said.

When asked directly if such secret jails may exist in Ukraine, Tkachuk said: "Nothing is impossible in life, unfortunately. For now, we say that we will carefully check this information."

The joint report by the rights groups also accuses Russia-backed separatists in eastern Ukraine of illegally imprisoning, abusing, and torturing people who live on the territory they control.

More than 9,500 civilians have died so far in the conflict, and international efforts to broker a peace have so far failed.