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At least two people were killed in a gunfight early on Sunday near a Ukrainian city controlled by pro-Russian separatists, shaking an already fragile international accord that was designed to avert a wider conflict.
After the deaths, Russia questioned whether Ukraine's Western-backed government was complying with the agreement, brokered last week in Geneva, to end a crisis that has made Russia's ties with the West more fraught than at any time since the Cold War.
The separatists said gunmen from Ukraine's Right Sector nationalist group had attacked them. The Right Sector denied any role, saying Russian special forces were behind the clash.
Failure of the Geneva agreement could bring more bloodshed in eastern Ukraine, but may also prompt the United States early next week to impose tougher sanctions on the Kremlin - with far-reaching potential consequences for many economies and for importers of Russian energy.
The deal signed in Geneva last week by the European Union, Russia, Ukraine and the United States agreed that illegal armed groups would go home.
So far, the pro-Russian militants have shown no signs of budging, though there was some hope of progress after Kiev said it would not move against the separatists over Easter, and international mediators headed to eastern Ukraine to try to persuade them to disarm.
But the shootings near Slaviansk - already a flashpoint for tensions between Ukraine's rival camps - are likely to make that task even harder, hardening the view of pro-Russian sections of society that they cannot trust Kiev.
"The Easter truce has been violated," the Russian foreign ministry said in a statement. "This provocation ... testifies to the lack of will on the part of the Kiev authorities to rein in and disarm nationalists and extremists." Right Sector spokesman Artem Skoropadsky said it was a "blasphemous provocation from Russia: blasphemous because it took place on a holy night for Christians, on Easter night. This was clearly carried out by Russian special forces."
