Russia's Federal Security Service (FSB) has detained a Russian citizen on suspicion of state treason after he sought to travel to Ukraine to join its armed forces, the TASS news agency reported on Tuesday.
The 35-year-old man from Russia's Kabardino-Balkaria republic in the North Caucasus had been detained while trying to leave for Ukraine via Belarus, the news agency said.
"It has been established that the detained person planned to join the ranks of one of the armed formations taking part in military actions on the territory of Ukraine against the armed forces of the Russian Federation," it quoted the FSB as saying.
The detention comes at a sensitive time when Ukraine is mounting a major counteroffensive in a bid to drive Russian forces from its territory.
Russian political background to the arrest
It also comes amid heightened tensions within Russia following an abortive mutiny at the weekend by mercenary fighters of the Wagner Group which has spearheaded some of Russia's military efforts in Ukraine.