Ukraine is starting a probe into last weekend's shooting near the town of Slaviansk which left three pro-Russian activists dead and there others wounded, Novinite reports.
The incident took place amid a declared Easter truce in Ukraine, following days of an "anti-terrorist" operation by Kiev forces against pro-Russian separatists.
Viktoriya Suimar from Ukraine's National Security Council was quoted by the BBC as saying that it was to early to tell who was responsible for the attack, but added that criminal groups could have been behind it.
According to the Interior Ministry, about 12 unknown men arrived at a roadblock set up by rebels near Slaviansk and opened fire at people guarding the checkpoint.
Russia earlier expressed outrage at the incident, putting the blame on nationalists from Ukraine's Right Sector.
Commenting on reports of the organization's involvement in the incident aired on Russian TV, a Right Sector spokesman described the claims as a "provocation" from the Russian secret services.