A reporter with a top Moscow opposition newspaper was briefly detained on June 16 in a separatist-held area in eastern Ukraine.
Pavel Kanygin was later expelled to Russia, his colleagues said, after apparently being beaten up.
Novaya Gazeta has been one of the only Russian media sources to investigate Western and Ukrainian accusations that the Kremlin has been both orchestrating and supporting the 14-month uprising in eastern Ukraine.
The paper's editor and several reporters said Kanygin was detained in a rebel-run part of the Ukrainian war zone and taken to a separatist-controlled village on the border with Russia.
"It seems like he was detained by the [rebels'] state security ministry," separatist military commander Eduard Basurin told the media by telephone.
A Russian Foreign Ministry spokeswoman said Moscow was aware of the reporter's detention and was seeking his immediate release.