Six people have been reported killed in a gun battle near Sloviansk in the worst violence in eastern Ukraine in seven weeks of protests against the pro-Western government in Kiev.

The deaths occurred during a night attack, presumably by anti-Russian radicals, on a checkpoint on the outskirts of Sloviansk in Donetsk region, protesters told Russian TV channels. A convoy of four SUVs drove by the checkpoint and opened fire on local residents who guarded the checkpoint.

The civilians at the checkpoint were unarmed because Kiev and the protesters had both declared an Easter truce.

Around 20 militiamen drove in from Sloviansk, killed three attackers and captured two of their cars, eyewitnesses said. The other assailants fled in the remaining two cars.

The Interior Ministry office in Donetsk confirmed that three civilians were killed and three others were wounded. It said twelve attackers escaped, taking with them their killed and wounded men, whose numbers were unknown.

Kiev said no military operations had been conducted in the region at the time of the incident. It described Sloviansk, which is controlled by the protesters, as “the most dangerous place in Ukraine.”

Russian television showed a medallion with paramilitary symbols of the neo-Nazi Right Sector group from western Ukraine, and a business card of its leader, Dmitry Yarosh, found in the captured cars along with a machine gun, a U.S.-made night vision device, bullet-proof vests and ammunition.

The Right Sector denied the group’s involvement in the attack and alleged it was a “provocation by Russian special forces.”

The Right Sector had been actively involved in violent protests in Kiev, which brought down the government of former President Viktor Yanukovych in February. Far right radicals were suspected of orchestrating the sniper attacks — killing about 100 protesters and police — during the Kiev violence that was later blamed on government forces.

Mr. Yarosh is running for Ukrainian president in the elections scheduled for May 25, while many Right Sector militants have joined the newly-formed National Guard, which has been deployed in eastern Ukraine for Kiev’s “anti-terrorist operation” against Russian-speaking protesters.

The leader of the protesters, Vyacheslav Ponomaryev, ordered a curfew in Sloviansk and called on Russia to send peacekeepers to protect local citizens from further attacks.

The Russian Foreign Ministry expressed outrage at the attack and demanded that Kiev deliver on its commitment to de-escalate the violence.

The deadly shootout occurred two days after Russia, the U.S., the European Union and Ukraine agreed on a roadmap to defuse tension in eastern Ukraine through disarmament of “illegal armed groups” and delegation of more power to Russian-speaking regions.

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