Moscow has expelled a Ukrainian diplomat in retaliation for Kiev's expulsion of a Russian diplomat, RIA news agency on Tuesday quoted a source at the Russian Foreign Ministry as saying.
It gave no further details.
Ties between Moscow and Kiev hit rock bottom over Russia's annexation of Ukraine's Crimea Peninsula in March 2014.
Unrest then started in Ukraine's Russian-speaking east and Kiev and the West accuse Moscow of driving a separatist rebellion there. Moscow denies that.