A court in Russia has upheld a lower-court decision to extend the detention of Yury Dmitriyev, a Russian historian and prominent Gulag researcher, who is being tried on charges of sexually assaulting his adopted daughter, which he and his supporters deny.

The Supreme Court of the northwestern region of Karelia on May 7 rejected Dmitriyev's appeal and remanded him in custody at least until June 25 while his trial continues behind closed doors.

A lower court extended Dmitriyev's detention until June 25 in late March.

Dmitriyev's supporters have said the charges were brought against him because of his research into a side of history that complicates the Kremlin's glorification of the Soviet past.