KYIV, Ukraine (AP) — Ukraine’s military said four of its soldiers were killed Friday during a mortar attack in the eastern region where its forces have been fighting Russian-backed separatists since 2014.

The attack in the hamlet of Shumi, reported by the Joint Forces Operation, was the most significant violation this year of a ceasefire — under which fighting has declined.

More than 13,000 people have died in the conflict.

The Ukrainian statement blamed the mortar attack on Russian troops. Russia has consistently denied having a military presence in the region.