The acting Ukrainian President, Oleksandr Turchynov, said yesterday that Russia had supported the torture and murder of a local politician in the east of the country who was loyal to Kiev.
The President called for the re-launch of an "anti-terrorist operation" against pro-Moscow separatists that was suspended only last week.
The remains of Vladimir Rybak, a town councillor and member of Yulia Tymoshenko's Our Ukraine Fatherland party, and an unidentified body, were believed to have been found in separatist-held Slaviansk.
"These crimes are being carried out with the full support and indulgence of the Russian Federation," said Turchynov. "I call on the security agencies to re-launch and carry out effective anti-terrorist measures, with the aim of protecting Ukrainian citizens living in east [Ukraine] from terrorists."
Ukraine's security forces had largely suspended what was a fairly limited operation to respond to the takeover of the eastern town by pro-Russian separatists after an accord with Moscow last week to try to defuse the crisis.
The announcement came as the Ukrainian Defence Ministry said one of its aircraft was hit by small arms fire while on a reconnaissance flight over Slaviansk.
The aircraft, a twin-engine propeller-driven Antonov An-30, landed safely after being struck several times by small arms fire, the ministry said. Tensions in eastern Ukraine have been rapidly spiralling out of control since three separatist activists were killed in a shoot-out on Monday.