UN Secretary-General Antonio Guterres said Wednesday that he will set up a fact-finding mission to look into last Friday's attack on a Donetsk prison that reportedly killed dozens of prisoners.

"I decided, in line with my own competencies and powers, to launch a fact-finding mission - I have not the authority to do criminal investigations, but (have the authority) to launch a fact-finding mission," he told reporters.

He said the terms of reference for the said mission are being prepared at the moment and will be shared with both Russia and Ukraine.

Ukraine and Russia have traded blame on a fatal attack on the prison in Olenivka that happened at the end of July. 

(With input from Xinhua)