A Ukrainian official has reported cease-fire violations in the early hours of Sunday, shortly after a truce went into effect to end intense fighting in the country's east between Ukrainian forces and pro-Russian separatists.
The head of Ukraine's security service, Valentyn Nalyvaichenko said Russian Cossacks appear to be responsible for mortar shelling in the Luhansk region. He said an investigation is under way.
Earlier, Ukraine's President Petro Poroshenko ordered his forces to comply with a cease-fire that went into effect at midnight.
Under the terms of the European deal brokered in Minsk, Ukrainian forces and Russian-backed separatist rebels were to stop shooting. If that holds, both sides are to begin pulling back their heavy weaponry to form a wide buffer zone.