Commandos have moved on the separatist stronghold of Sloviansk in eastern Ukraine days after a new "anti-terrorist" operation was announced.
A number of casualties were reported among the separatists as the commandos, backed by armour, cleared barricades near the town.
Separatists are occupying key buildings in at least a dozen eastern towns.
Russian President Vladimir Putin warned of "consequences" if Kiev used the army against its own people.
Speaking on Russian TV as news of the Sloviansk operation was coming in, Mr Putin said: "If the regime in Kiev has begun using the army against the population inside the country, then this is undoubtedly a very serious crime."
Silly Ukrainians, you're only supposed to use your army to go into another country undercover and steal someone else's provinces!
If you use your army against your own people, you might get run out of town on a rail by violent protests in the capital that force the government leaders to flee and seek refuge in a friendly neighboring country while the opposition claims victory and the people start to fragment along ethnic lines.