Ukraine is holding a day of mourning after 13 people were killed when a bus was hit by what the government says was a shell fired by Russian-backed separatists.

President Petro Poroshenko said respects would be paid Thursday for all people killed by rebel military offensives in eastern Ukraine.

Separatists deny responsibility for the deaths and accuse Ukrainian forces of staging an attack near the town of Volnovakha to smear them. Russia media has speculated the blast was provoked by an anti-personnel mine.

Surveillance camera footage filmed during Tuesday's incident shows shells falling near the bus, although the vehicle is not seen in shot. Monitors from the Organization for Security and Cooperation in Europe said the bus showed "damage consistent with a nearby rocket impact."