KIEV: Ukraine held a day of mourning on Thursday after 13 people were killed two days ago when their bus was hit by what the government says was a shell fired by Russian-backed separatists.

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

The separatists deny responsibility for the deaths and accuse Ukrainian forces of staging an attack near the town of Volnovakha in a bid to smear them. Russia media have speculated the blast was from an anti-personnel mine.

Monitors from the Organization for Security and Cooperation in Europe said the bus showed “damage consistent with a nearby rocket impact.” Surveillance camera footage of Tuesday’s deadly attack shows shells falling near the government checkpoint where the bus was hit, although the vehicle is not seen.

The OSCE said a group of Russian and Ukrainian military officials and separatist representatives will investigate the deadly attack.