At least three people were killed in a gunfight in the early hours of Sunday near a Ukrainian city controlled by pro-Russian separatists, shaking an already fragile international accord that was designed to avert a wider conflict.
The incident triggered a war of words between Moscow and Ukraine's Western-backed government, with each questioning the other's compliance 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 armed men from Ukraine's Right Sector nationalist group had attacked them. The group denied any role, saying Russian special forces were behind the shootings.