The chairman of Belarus's KGB says investigations have been launched into the activities of 138 Belarusian citizens who are thought to have been combatants in Ukraine's eastern regions of Luhansk and Donetsk.

KGB chief Valeriy Vakulchik said on June 23 that the suspects are thought to have fought alongside Ukrainian government forces as well as on the side of Russia-backed separatists.

Vakulchik said all 138 suspects will be charged with fighting in a foreign country as a hired mercenary.

In April, Belarus adopted a law that made it illegal for Belarusians to fight as mercenaries in foreign military conflicts.

Dozens of Belarusian citizens, along with hundreds of citizens from other former Soviet republics, have joined Russia-backed separatists or Ukrainian government forces as mercenary fighters in eastern Ukraine since Moscow's illegal annexation of Ukraine's Crimea region in 2014.

More than 9,300 people have been killed in the conflict in eastern Ukraine since April 2014.

Based on reporting by TASS and Interfax