KIEV, Ukraine—Russia’s foreign ministry accused Ukrainian authorities today of failing to rein in armed extremists after a bloody shootout shattered what was supposed to be an Easter truce. At least one pro-Russian separatist was killed in an apparent early-morning attack on a checkpoint on the outskirts of the eastern Ukraine town of Slovyansk.

Some reports suggest the attack may have left as many as four separatists dead, but local police are confirming one killed and three wounded. The separatists are blaming the incident at the checkpoint they had set up outside Slovyansk on the Ukrainian ultranationalists from the Right Sector, a group that was prominent in the months-long uprising against ousted President Viktor Yanukovych.

Reports from the scene are confused, with separatist fighters saying four cars approached the checkpoint 18 kilometers outside Slovyansk, one of 10 cities in the mostly Russian-speaking east of the country where pro-Russian militants have seized government buildings. The gunmen immediately started opening fire, according to these reports.

Two Reuters journalists at the scene reported seeing two dead bodies, one with gunshot wounds to the head. One of the dead was wearing combat fatigues and the other was in civilian clothes. Both bodies were lying in the back of a truck, partially covered by a blanket.

Within hours of the incident—the attack occurred at around one o’clock this morning—the Russian foreign ministry issued a statement saying, “Russia is indignant about this provocation by gunmen, which testifies to the lack of will on the part of the Kiev authorities to rein in and disarm nationalists and extremists.”

Ukraine’s police and intelligence service counter-charged, accusing Russia of staging the deadly shooting. Viktoria Syumar, a deputy head of the National Security and Defense Council in Kiev, claimed in a Facebook posting that Russia is preparing the grounds to invade Ukraine. And the country’s intelligence service, the SBU, argued the attack was staged. “Armed lawbreakers and saboteurs who are terrorizing the local population around Slaviansk have turned to cynical provocation,” it said in a statement.

But later Ukraine’s Interior Ministry admitted at least three people had died, and that two were locals. One of them was a 60-year-old bus driver, who had joined up with local separatists the first day pro-Russian militants had erected barricades, the English-language Kyiv Post newspaper reported.

According to the Interior Ministry some assailants may have been killed as well. It said in a statement the attackers sped off in two cars, taking with them dead and wounded.

The assault came just hours after top mediators from the Organization for Security and Co-operation in Europe (OSCE) arrived in nearby Donetsk to hold talks with pro-Russian activists in a bid to persuade separatists to disarm and to leave the government buildings they are occupying.

The gun battle will complicate the OSCE mission. Armed separatists occupying public buildings in Donetsk and other Russian-speaking border towns are refusing to recognize an agreement reached in Geneva on Thursday by diplomats from Ukraine, Russia, the United States, and the European Union. Under the accord, the OSCE is tasked with overseeing the disarmament of militants and their evacuation from occupied buildings. Russia denies Ukrainian and Western claims that it is fomenting pro-Russian unrest in eastern Ukraine.

Video footage from the scene of the gun battle carried on Russian state television, which was the first media organization to report the incident, showed two burned-out cars, both riddled with bullet holes. Nearby there was a body on the ground under a sheet.

A separatist told Russian television that when the convoy of four cars pulled up, “one of our men approached them. They shot him in the head and he fell immediately. There was some sniper fire.” He claimed: “We had three dead and four wounded.” He also claimed separatists called in reinforcements and managed to kill two of the assailants. His claims, though, have not been verified by independent media sources or by local officials.

An OSCE official contacted by phone told The Daily Beast that he was unable to confirm any details about the clash. But he conceded that whatever the truth of the matter, the incident would make the OSCE task much harder. Swiss envoy Christian Schoenenberger, the formal head of the OSCE mission, acknowledged to reporters Saturday, before this shooting, that his mediators face daunting challenges.

Schoenenberger said his monitors had spoken to several pro-Russian activists: “For the time being the political will is not there to move out,” he said. “That’s the task of the monitors, to create this political will, inform the people, so eventually they will understand that the best option for them is to move out,” he told reporters.

The bitter divisions were on display this Easter Sunday even before the reports of the clash in Slovyansk, with competing Orthodox Easter messages from patriarchs in Kiev and Moscow. The head of Ukraine’s Orthodox Church Patriarch Filaret, whose church backed Yanukovych ‘s ouster, accused Russia of “aggression” and “evil.” Russian Church Patriarch Kirill asked God to intervene to end the machinations of those who wanted to separate Russia and Ukraine.

But the intervention on the minds of most Ukrainians following this morning’s gun battle was whether the incident would prompt a greater one, not from God, but from Vladimir Putin.