BRUSSELS, April 13 (Reuters) - NATO's leader voiced concern on Sunday over the appearance of pro-Russian gunmen in eastern Ukraine and warned that any further Russian military interference in the country would deepen Moscow's international isolation.
Ukraine's government sent security forces to the eastern city of Slaviansk on Sunday where pro-Russian militants have seized control. One Ukrainian state security officer was killed and five others wounded during the operation, the interior minister said.
"I am extremely concerned about the further escalation of tension in eastern Ukraine," NATO Secretary-General Anders Fogh Rasmussen said in a statement.
"We see a concerted campaign of violence by pro-Russian separatists, aiming to destabilise Ukraine as a sovereign state."
Rasmussen said the appearance in eastern Ukraine of men with specialised Russian weapons and identical uniforms without insignia, as previously worn by Russian troops during Russia's seizure of Crimea, was a "grave development".
He called 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.
"Any further Russian military interference, under any pretext, will only deepen Russia's international isolation," he said. (Reporting by Adrian Croft; Editing by Raissa Kasolowsky)