Several armoured personnel carriers have reportedly entered the eastern Ukrainian town of Slaviansk, with the lead vehicle showing the Russian flag, according to Reuters.
There are reported to be five or six vehicles, each carrying around 15 armed men, who are waving to local people.
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.