MOSCOW, Dec 27 (Reuters) - A Russian court on Monday added two more years to a 13-year jail sentence for historian Yuri Dmitriev, in a sex abuse case that his supporters say was trumped up to punish him for uncovering mass graves of Stalin"s Gulags. Dmitriev, who worked with prominent rights group Memorial documenting Soviet-era repression, was found guilty in July last year of sexually abusing his adopted daughter, a charge he denied. Russia"s Supreme Court in October refused to consider his appeal. Dmitriev was initially sentenced to 3-1/2 years and due to be freed in November 2020 because of time served. But weeks before his release the Petrozavodsk city court in Russia"s northwestern Karelia region abruptly added a decade to his term. In a statement on its website on Monday, the court said the sentence had been extended by a further two years and Dmitriev would be held in a high-security penal colony. Dmitriev"s lawyer said he would appeal the ruling, independent news outlet Sota Vision reported. Dmitriev"s supporters say his case is retribution for exposing Stalin"s crimes, including the 1937-1938 Great Terror when officials estimate at least 700,000 people were executed. Dmitriev found a mass grave containing thousands of bodies of people held in the Gulags, the Soviet prison camp network. The Soviet state disavowed Stalin"s repressions after the dictator"s death in 1953, but some still revere him for leading the country during its World War Two victory. Rights campaigners accuse Russia"s authorities of blocking efforts to account fully for Soviet repression. Memorial, a rights group founded by Soviet-era dissidents, has said the accusations against Dmitriev were groundless. Memorial itself faces the threat of being shut down at the behest of state prosecutors who accuse it of disobeying laws requiring groups to register as "foreign agents". read more The U.S. embassy last year condemned Dmitriev"s long jail term, describing it as a setback for human rights and historical truth in Russia. The Kremlin has said it is not involved in his case.