Ukraine on Friday accused Russian forces of violating international law by abducting the mayor of Melitopol, a city in southeastern Ukraine that fell under Russian control during the invasion.
Russia has not commented on the fate of Mayor Ivan Fedorov. Ukraine says Russian forces kidnapped him after falsely accusing him of terrorism.
"The abduction of the mayor of Melitopol is classified as a war crime under the Geneva Conventions and the Additional Protocol, which prohibit the taking of civilian hostages during the war," Ukraine's foreign ministry said in a statement.
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Meanwhile, Ukrainian President Volodymyr Zelensky called on the leaders of France and Germany to help secure the release of the mayor of Melitopol.
"During the night and today we are talking to our partners about the situation with our mayor. Our demand is clear: he must be released immediately… I have already phoned (German) Chancellor Olaf Scholz. I have spoken to (French) President Emmanuel Macron… I will speak to all the necessary people to get our people released," Zelensky said in a video released by the Ukrainian presidency.
"We expect world leaders to show us how they can influence the situation."
According to the Ukrainian president and parliament, mayor Ivan Fedorov was abducted on Friday by Russian soldiers occupying Melitopol, a town in southern Ukraine halfway between Mariupol and Kherson, because he "refused to cooperate with the enemy."
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According to the Ukrainian parliament, the mayor was arrested while at the city's crisis centre tackling supply issues.
Zelensky said 2,000 Ukrainians demonstrated in Melitopol on Saturday against the Russian invasion and to demand the release of their mayor.
"Do you hear, Moscow? If 2,000 people demonstrate in Melitopol against the occupation, how many are there in Moscow against the war?" he said in his video.
Before the Russian invasion began on 24 February, Melitopol had a population of just over 150,000.