Ukrainian forces have launched an operation against pro-Russian separatists in their rebel stronghold of Slovyansk.

Two Ukrainian helicopters have been shot down, with two pilots killed and another injured, according to the Defence Ministry in Kyiv.

Russian TV showed pictures of a third pilot apparently injured and captured.

The Ukrainian security service (SBU) says shoulder-launched anti-aircraft missiles have been used, proving that pro-Russian forces include foreign-trained specialists.

One rebel among the group holding the captured pilot claimed the separatists had saved him.

“He was abandoned by his colleagues, they took his pistol and ran away,” the man said.

Ukraine’s Interior Minister Arsen Avakov said on Facebook that Slovyansk was surrounded. Pro-Russian separatists had set up barricades on the outskirts of the town.

Within Slovyansk’s boundaries, the minister claimed all ten separatist checkpoints had been destroyed and several rebels captured.

Russia has asked the Organisation for Security and Cooperation in Europe (OSCE) to intervene to stop what it calls the “raid of retaliation” in the east.

Moscow had threatened catastrophic consequences should Ukraine move against the separatists, prompting fears it might send its troops across the border.