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...

Be careful when reading news articles about the war. This one makes it seem like he was executed without trial. Here's another one that paints it in a different light:

https://interfax.com/newsroom/top-stories/75617/

Seems like he tried to escape arrest and was shot. There's probably a lot more to this story than either of these articles show.