Pro-Russian rebels have reportedly entered a strategically important town on Ukraine's south-east coast.
The move comes two days after the presidents of both nations met for peace talks.
Russia denies any direct link to the separatists.
Rebel leader Alexander Zakharachenko says there are some Russians working with his men, but they are doing it out of choice, and not by political direction.
His words have been translated: "Among volunteers from Russia there have always been many retired military servicemen. They fight alongside us, understanding that this is their duty.
"There are currently serving Russian soldiers among us who prefer to spend their vacations, not on sea beaches, but among us, fighting for their freedom."