London, Apr 25 (IANS): An advisor to Ukraine President Volodymyr Zelensky has compared Russian troops to ISIS militants after Russian forces allegedly threatened to execute a captured soldier on video unless his mother paid a ransom, Daily Mail reported.

Ukrainian soldier Novikov Alexey Antonovich from Mariupol said he was captured on April 23, and was part of the 109th brigade of the Donetsk territorial defence.

He added that the Russian soldiers threatened to send a video of his execution if they didn't receive the money.

Footage showed the Ukrainian soldier being filmed as he was asked his name and how he was being treated, Daily Mail reported.

The video was sent to the young soldier's mother with a demand of 5,000 euros if she wanted her son to remain alive.

In the footage shared online by Podolyak, the Ukrainian soldier told his Russian captor he was being treated 'okay'. He said: "I have food here, water, a toilet."

Russian President Vladimir Putin wanted his war in Ukraine to last just days and end with the toppling of its pro-Western government.

However, the war entered its third month last week as Russia continued to batter the southern port city of Mariupol and launched fresh attacks throughout the Donbas region.