KIEV, April 20 -- Russia should be responsible for a fatal shootout between unidentified gunmen and pro-Moscow activists in the eastern Ukrainian town of Slavyansk, authorities said Sunday.

"Armed lawbreakers and saboteurs who are terrorizing the local population around Slavyansk ... have turned to cynical provocation," the Ukrainian Security Service said in a statement.

Describing the incident as a "staged attack," the statement said no group was present "other than the saboteurs and crime figures, supported and armed by officers of Russia's GRU (military intelligence)."

Meanwhile, the Right Sector, a Ukrainian nationalist group blamed by Moscow and pro-Russia activists for the shooting, denied involvement and also accused Russia's special forces of staging the attack.

The group, which was at the vanguard of street protests that forced the ouster of President Viktor Yanukovych in February, said the charge against it was "lies" and "propaganda" designed to portray eastern Ukraine as ungovernable by Kiev.

Earlier in the day, Russia's state television reported that a group of unidentified gunmen attacked a checkpoint controlled by pro-Russia activists, resulting in the death of three activists and two gunmen.

The state-run Rossiya 24 news station later revised its report to one dead and two wounded on the pro-Russia side and up to seven killed or wounded among the attackers.

The deaths were the first in armed clashes in eastern Ukraine since the United States, Russia, Ukraine and the European Union signed an accord in Geneva on Thursday, asking all sides to refrain from violence, intimidation or provocative actions as part of initial steps to restore order in the country.

Tensions in eastern Ukraine have been escalating since last weekend following an armed confrontation between Ukrainian security forces and pro-Moscow activists, who demanded a referendum on autonomy and closer ties with Russia.