This was reported by Censor.NET citing the Voice of America.
Volker said meeting with Akhtem Chiygoz and Ilmi Umerov that their release was a minor positive signal.
Earlier, the U.S. Special Envoy to Ukraine held talks with Kremlin representative Vladislav Surkov describing the meeting as constructive but showing no tangible results. The U.S. official said they were trying to understand whether it was possible to find a common ground on how peacekeepers could be used to resolve the conflict.
He believes this step can be effective only if peacekeepers are endowed with broad powers and will be able to operate throughout the region.
Volker also accused Russia of supporting the ongoing conflict in the Donbas, noting however that Moscow started realizing that the continuation of the aggressive policy in the eastern Ukraine was at odds the Kremlin's interests.
He also said that the United States was actively discussing the possibility of providing Kyiv with military equipment and lethal weapons for protection of its territory.
The U.S. official says the issue is not as controversial as it may seem since the Ukrainian authorities require these weapons solely for the country's defense.
Earlier, Umerov and Chiygoz were released by Russia and extradited from Crimea to Turkey.
In 2016, FSB investigators initiated a criminal case against Crimean Tatar, deputy head of Mejlis and participant of Crimean Tatars national liberation movement Ilmi Umerov. He is charged with public calls to violate the territorial integrity of Russia.
Umerov's detention, search and the criminal case, and his keeping in psychiatric hospital and pushing through forced psychiatric expertise stirred global response.
Simferopol city court controlled by the Kremlin started hearing the case against Umerov on June 7, 2017. Umerov believes his case is politically charged.
On Sept. 11, the Supreme Court of Russia-occupied Crimea sentenced Chiygoz to eight years in prison.
On Aug. 2, 2016 Russian-controlled Supreme Court of Crimea started to consider materially "the case of Feb. 26" against Akhtem Chiygoz. Earlier, on July 20, 2016, during hearing in camera, the court divided "the case of Feb. 26" into two separate cases: one against Akhtem Chiygoz and another against the rest of the defendants.
After Crimea was occupied by Russia, Kremlin puppets started trials against Crimean Tatar activists who participated in protests in support of Ukraine's territorial integrity. The events took place on Feb. 26, 2014. According to occupant authorities, two protesters were killed in a stampede, 79 people received injuries. Those detained in connection with the case - Akhtem Chiygoz, Ali Asanov, and Mustafa Degermendzhy - were named political prisoners by Russian Memorial human rights center.