At least 13 people have been killed in a horrific bus attack in central Donetsk in Ukraine on Thursday, as pro-Russian rebels seize greater control of the city.

A dozen people were killed when an explosion ripped through the bus, in one of the deadliest days in the separatist war between the Ukrainian government and pro-Russian rebels.

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Ukrainian troops also withdrew from defending Donetsk’s main airport, with six soldiers killed and another 16 wounded, according to the BBC.

Pro-Russian rebels paraded 20 captured Ukrainian soldiers through Donetsk following the bus attack, and forced them to kneel while enraged locals threw glass at them, some of it from the bus blast.

Stunned residents gathered around the shattered bus as victims remained in their seats hours after the attack.

Both the Russian and Ukrainian government have blamed each other for the blast, with a peace treaty signed between the two countries just days before failing to have impact.

Ukrainian prime minister Arseniy Yatsenyuk blamed Moscow for the attack, accusing pro-Russian rebels of committing “a horrible act against humanity”.

“And responsibility for this is borne by the Russian Federation,” Mr Yatsenyuk said.

Russia said it would open an investigation into the shelling, but said responsibility lied with the Ukrainian government in Kiev.

Russian foreign minister Sergei Lavrov called the attack “a monstrous crime” whose ultimate responsibility rested with “‘the party of war’ in Kiev and its foreign sponsors”.