NATO chief Anders Fogh Rasmussen said Sunday he was "extremely concerned" about the escalation of tensions in east Ukraine, saying actions by uniformed, pro-Russian gunmen with military weapons pointed to an organised campaign to destabilise the region.
Drawing parallels with Crimea, the Ukrainian region Russia annexed last month after deploying forces there, he warned Moscow against further military interference.
"We see a concerted campaign of violence by pro-Russian separatists, aiming to destabilise Ukraine as a sovereign state," Fogh Rasmussen said in a statement.
"The reappearance of men with specialised Russian weapons and identical uniforms without insignia, as previously worn by Russian troops during Russia's illegal and illegitimate seizure of Crimea, is a grave development.
"I call on Russia to de-escalate the crisis and pull back its large number of troops, including special forces, from the area around Ukraine's border," he said.
"Any further Russian military interference, under any pretext, will only deepen Russia's international isolation," he warned.
The government in Ukraine launched a special forces operation early on Sunday to oust pro-Russian gunmen who had taken control of a police station in the eastern city of Slavyansk the day before.
Interior Minister Arsen Avakov said one Ukrainian special forces officer was killed and five were wounded in a firefight. He asserted that there were also dead and wounded on the other side but could not give figures.