ST PETERSBURG/MOSCOW (Reuters) - A Russian court on Tuesday rejected an appeal by Yuri Dmitriev, a historian who exposed Stalin-era crimes, ordering him to be kept in jail to serve out a 13-year sentence that his supporters say was based on fabricated charges.

Dmitriev, 65, was found guilty in July of sexually abusing his adopted daughter, a charge he denied. His lawyer said they would now take their appeal against the sentence to Russia’s Supreme Court.

Dmitriev was due to be freed in November due to time served. But a court in Russia’s northwestern Karelia region in September abruptly added a decade to his sentence and said he would be held in a high-security penal colony.

His supporters say the case against him is retribution for him helping expose Stalin’s 1937-38 Great Terror, in which nearly 700,000 people were executed, according to conservative official estimates.

Dmitriev found a mass grave after the Soviet breakup containing thousands of bodies of people held in Stalin’s Gulag network of prison camps.

Memorial, a rights group where Dmitriev works, has said the accusations against him were groundless.