As global outrage grew over an explosion that killed at least 50 Ukrainian prisoners held at a Russian detention camp, Ukrainian authorities called for an international investigation Saturday while marshaling evidence that they said would prove that Russia had orchestrated what they described as a "terrorist attack.”

Since the explosion late Thursday at Correctional Colony No. 120, a prison camp in the Russian-occupied eastern region of Donetsk, the warring parties have presented diametrically opposed accounts of what happened, further embittering a war now entering its sixth month.

Russian officials claimed that Ukrainians, using precision weapons supplied by the United States, had attacked the prison themselves, to deter defectors. Ukrainian authorities rejected the narrative as absurd and said the deaths were a premeditated atrocity committed by Russian forces from within the prison, where survivors described being given just enough food to survive and suffering ritual beatings, including with chains and metal pipes.