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Ukraine's president says two prominent Crimean Tatar leaders who were imprisoned in Russian-occupied Crimea have been released and flown to Turkey.

President Petro Poroshenko in a tweet on Wednesday thanked the Turkish president for his help mediating the release of Ilmi Umerov and Akhtem Chiygoz.

Chiygoz, the deputy chairman of the Mejlis representative body for the Tatars in Crimea, had been sentenced to eight years in prison on a charge of organizing a riot during the 2014 Russian annexation of the peninsula. Russia sent troops to Crimea and declared the annexation about three weeks later.

Umerov, another deputy chairman of the Mejlis, had been sentenced to two years in prison for his criticism of the annexation.

Crimea's Supreme Court effectively banned the Mejlis in April, declaring it an extremist organization.