Kyiv Mayor Vitali Klitschko warned Saturday that an imposter used "deep fake" technology to conduct video calls and fool the mayors of Vienna, Berlin and Madrid into believing they were speaking with the Ukrainian capital's real mayor.

"Several mayors in Europe have been contacted by a fake mayor of Kyiv who has been saying absurd things," Klitschko told the German daily newspaper Bild. "This is criminal energy. It must be urgently investigated who is behind it."

The office of Berlin Mayor Franziska Giffey tweeted Friday night that she cut short a call with the reputed Kyiv mayor after his comments and questions made her suspicious. "The course of the conversation and the setting of topics" made Giffey wary, her office said without elaborating.

At the beginning of the conversation, Giffey said she was asked if the talk could be held in Russian and translated, she said in the tweet. She said she paid relatively little attention to the voice of her conversation partner, as she was following the translation.

The mayor's administrative staff later said in a tweet that Ukraine's ambassador to Germany, Andriy Melnyk, confirmed that Giffey had not been talking to Klitschko.

The office published a photo that showed both the German capital's mayor and the fake Klitschko on a big screen. It said that initially, "there was no evidence that the video conference was not conducted with a real person. To all appearances, it is deep fake."

"Deep fakes" can take the form of technically sophisticated videos that appear to realistically depict the speech and actions of a person in real life.

Police were investigating the incident, Giffey’s office said.

Madrid Mayor Jose Luis Martínez-Almeida also interrupted a video call with someone claiming to be Klitschko on Friday. The mayor of Spain's capital suspected he wasn't speaking with his Kyiv counterpart and has filed a complaint with the police.

Vienna Mayor Michael Ludwig did not end his call with the imposter earlier this week because he didn't notice any suspicious behavior, Austrian public broadcaster ORF reported.

Ludwig had received the invitation to the call from Kyiv, which was supposedly a thank you for aid sent from Vienna, Ludwig's spokesperson told Deutsche Presse-Agentur (dpa), adding this was nothing unusual.

"The conversation was conducted in English, which was admittedly somewhat strange," the spokesperson also said.

"Since no tricky topics were discussed in the conversation, this is certainly annoying in the specific occasion but not a big problem," Ludwig said.

The man he thought was the Kyiv mayor became unusually demanding toward the end of the call, Ludwig told the ORF broadcaster. "But it would not have made me now somehow question that," he said.

It was not clear who was behind the calls or what means were used to try to make the mayors think they were communicating with Klitschko.

On Saturday, Berlin's mayor said her encounter with the fraudster means that "in the future we will have to be even more scrutinizing, even more suspicious."

She called the use of a phony Klitschko "a means of modern warfare," referring to Russia's four-month war on Ukraine.

Kyiv's actual mayor also made a link to Russia's war in Ukraine.

"Friends! The enemy does not let up and is waging war on all fronts – in particular by disinformation, by discrediting Ukrainian politicians," Klitschko said in a post on the Telegram messaging app. "In order to quarrel with European partners, so that Ukraine would not be helped."

In his remarks to Bild, he warned other European officials to be careful if they are contacted by someone claiming to represent his office. "Please be careful in the future how appointments are arranged by me. Official conversations only come through official channels," he said.