Armed men have seized the regional government building and parliament in Crimea, according to Russian news agency Interfax.
It said local Tatar leader Refat Chubarov posted on Facebook, saying: “I have been told that the buildings of parliament and the council of ministers have been occupied by armed men in uniforms that do not bear any recognisable insignia.
“They have not yet made any demands.”
On Wednesday there was a confrontation between pro-Russia separatists and opposition supporters in Simferopol, Crimea, where the parliament building is located.
It came as Russia confirmed it will take part in talks on an International Monetary Fund (IMF) financial package for Ukraine.
Debt-hit Ukraine has said it needs £21bn ($35bn) over the next two years to avoid bankruptcy.
The European Commission says Russia should be included in efforts to provide financial assistance.
“All I can say is that we will take part in discussions (at the IMF), there is no talk of standing aside,” said Andrei Bokarev, head of the finance ministry’s international relations department.