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.