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.
A truce called in early December between government and separatists forces held for several weeks, but has unraveled in the past week. Ukrainian troop positions and residential areas under government control in the east came under artillery fire 129 times in the last day, the government said Thursday.
Attacks on the government-held airport building in the separatist stronghold of Donetsk have been especially intense. Villages nearby have also been hit by rebel fire aimed at government forces seeking to hold the terminal.
In an indication that Ukraine is bracing for a long-term commitment to the conflict, parliament on Thursday approved a presidential decree for three waves of military mobilization this year. The first call-up is to start Tuesday and last for 90 days.
Poroshenko said the need for mobilization was motivated by the worsening security situation.
Military officials would not state the number of troops called up in mobilizations.
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