By Olga Robinson, Shayan Sardarizadeh and Adam Robinson
BBC News
Ukrainian President Volodymyr Zelensky has called on world leaders to respond after a video emerged apparently showing a Ukrainian soldier being beheaded by a Russian serviceman.
"Everyone must react. Every leader. Do not wait for this to be forgotten," he said in a video address.
Ukraine's SBU security service said it was investigating the "war crime".
The Kremlin said the video was "awful" but its authenticity had to be checked as well as who was behind it.
The grainy and extremely graphic video appears to have been filmed on a mobile phone, and possibly during the summer months. It shows a man in military uniform wearing a yellow armband - frequently worn as identifying symbols by Ukrainian soldiers.
The perpetrator and other men visible in the clip have white bands on their legs, which Russian soldiers are known to wear as a means of identification.
They can also be heard to speak Russian, although that is not conclusive as many Ukrainians speak Russian too.
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