An attack on a passenger bus in Ukraine left nearly a dozen people dead as gunfire exchange between government soldiers and separatist forces around the Donetsk became more frequent, authorities said.

Ukrainian President Petro Poroshenko described the violence that has killed 11 people as a development that sent chills to the heart and he blamed separatists from Luhanks and Donetsk for the attack.

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A regional Ukrainian official said rebels swooped down on the bus using rocket launchers while they were carrying civilians through a checkpoint.

The conflict follows a decision by Germany, France, Russia and the Ukraine not to meet anymore in a summit in Kazakhstan this week because of violations to an existing ceasefire agreement.

The Organization for Security and Cooperation in Europe has called for all countries involved to exercise maximum restraint. The conflict has killed about 4,700 people since it erupted in April last year.

The clash between pro-Russian rebels and Ukrainian forces erupted following Russia's annexation of Crimea last year. The move led to the worst East-West ties since the Cold War. The United States and European nations point out Russia is supporting the separatists, by deploying more troops, which Moscow denies.

The bus was peppered by holes while stray blood stained the road near Volnovakha.

Meanwhile, shelling on Donetsk destroyed a big part of the airport's control tower. The complex housing the Sergei Prokofiev airport has disintegrated after months of fire.

After government forces attacked with missile launchers, the rebels started firing from tanks on the airport's new terminal, which was still held by Ukrainian forces.

The Ukrainian military spokesman said terrorists and the Russian military have chosen the tactic of escalating tensions. A Ukrainian fighter was killed and 10 hurt in the fighting.

TagsUkraine, Ukraine violence, Ukraine-Russia, bus attack.Donetsk