The CNN and BBC sites are covering it, I just want to point toward the ledes:

CNN

Ukraine's Interior Minister Arsen Avakov posted to Facebook Sunday: "Antiterrorist operation has started in Slaviansk. It is managed by Anti-terrorist Center of the Security Service of Ukraine. All the law enforcement agencies of the country are participating. God speed!"

BBC

Ukrainian forces have launched an operation against pro-Russian activists who seized a police station on Saturday, the interior minister says.

Arsen Avakov announced on his Facebook page that "all security units" were involved in an "anti-terror operation" in the eastern city of Sloviansk.

Note: The CNN quote was timestamped at 2:28 AM. The 2:45 AM update rephrased it.