Russia's Foreign Ministry said Wednesday that fighters from Syria and Libya were being deployed to the Nagorno-Karabakh conflict and that it was "deeply concerned" by the development.
"Fighters of illegal armed groups including from Syria and Libya are being deployed to the Nagorny Karabakh conflict zone in order to directly take part in fighting," the Foreign Ministry said.
"We are deeply concerned by these processes which not only escalate tensions in the conflict zone further but create long-lasting threats to security for all countries in the region."
Armenian and Azerbaijani forces are engaged in the heaviest fighting in years over Karabakh, an ethnic Armenian province that broke away from Azerbaijan in the 1990s during the collapse of the Soviet Union.
Armenia has accused Azerbaijan's ally Turkey of sending "mercenaries" to Karabakh.
The Russian statement made no mention of Turkey, but called on the "leadership of interested states" to take measures to prevent "foreign terrorists and mercenaries" from being involved in the conflict.