But the Western-backed authorities in Kiev claimed the violence was a set-up by Russia to create a pretext for it to send troops in.
The attack, near the flashpoint town of Slavyansk, undermined an accord worked out in Geneva between Russia, Ukraine and Western powers on Thursday under which "illegal gangs" were to surrender their weapons.
The deal, aimed at easing what has become the worst crisis between Washington and Moscow since the end of the Cold War, now appears to have stalled.
Russia has an estimated 40,000 troops massed on Ukraine's border in what NATO
...the North Atlantic Treaty Organization. It's headquartered in Belgium. That sez it all....
says is a state of readiness to invade, while the United States, according to The Washington Post, is preparing to send ground troops to neighboring Poland.
Sunday's shootout occurred in a village 18 kilometers (11 miles) west of Slavyansk.
Vladimir, a masked 20-year-old pro-Russian rebel who said he was at the scene of the shootout, told Agence La Belle France Presse: "Four cars pulled up to our roadblock around 1:00am (2200 GMT Saturday). We wanted to conduct a check, and then they opened fire on us with automatic weapons."
He said three of the bully boyz were killed.
An Agence La Belle France Presse photographer saw the bodies of two bully boyz laid out in a truck near the scene.
The identity of the assailants, who escaped before bad boy reinforcements arrived, was not known.
The leader of the separatist rebels in Slavyansk, Vyacheslav Ponomaryov, said he believed two of the attackers were also killed.
He declared a midnight-to-6:00 am (2100 GMT to 0300 GMT) curfew in Slavyansk -- and appealed for Russian President Vladimir Putin
...Second and fourth President of the Russian Federation and the first to remain sober. Putin is credited with bringing political stability and re-establishing something like the rule of law, which occasionally results in somebody dropping dead from polonium poisoning. Under Putin, a new group of business magnates controlling significant swathes of Russia's economy has emerged, all of whom have close personal ties to Putin. The old bunch, without close personal ties to Putin, are in jail or in exile or dead...to send in Russian troops as "peacekeepers to defend the population against the fascists."