This publication (based in Bulgaria) seems to publish fairly pro-Russian articles, which should be a red flag for news like this. A quick search for Kireev's name returns a lot of contradictory information. I found an article from the Sun [0] (UK-based, not sure how reliable) and from an Indian paper [1] that seem to confirm the news. I can't really comment on the reliability of any of these papers, not being familiar with any of them.
[0] https://www.thesun.co.uk/news/17857369/ukraine-peace-negotia...
[1] https://www.ibtimes.co.in/ukrainian-delegation-member-killed...