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UNITED NATIONS (AA) - UN Secretary-General Antonio Guterres is ''horrified'' by a video that shows the brutal execution of a Ukrainian prisoner of war and he demanded that the perpetrators be held accountable, his spokesperson said Wednesday.

''Regrettably, this is not an isolated incident, '' Stephane Dujarric told reporters.

He said the UN human rights monitoring mission in Ukraine is ''appalled by these particularly gruesome'' videos circulating on social media and it documented several violations of international humanitarian law, including those committed against prisoners of war in its recent reports.

''The slightest violation must be properly investigated, perpetrators must be held accountable,'' he said.

Ukrainian President Volodymyr Zelenskyy condemned the beheading and vowed to punish the perpetrators.

Foreign Minister Dmytro Kuleba compared Russia to Daesh and said Russia was worse than the terror group.

Kremlin spokesman Dmitry Peskov said the authenticity of the video must be checked before reaching any conclusions. ''Of course, these are terrible shots,'' he said.