In Russia, the government said its app had also been targeted by “a massive cyberattack”, which sent “fake information to the public”.
A purported screenshot of a fake alert was shared on Telegram by Atesh, a partisan Ukrainian group operating inside Russian-held territory.
It warned some users that “nuclear warheads mounted on P1-Suns [a Ukrainian attack drone] had been launched at all regions of Russia”.
The alert, which included profanities, opened with a message telling residents there was a “risk of [things] getting lively” and told them to “stock up urgently on food for three weeks and hide”.
On Monday morning there was no sense the messages had caused widespread panic in Russia and the incident was not being prominently featured on media outlets.
Asked about the breach, Kremlin spokesman Dmitry Peskov did not address it specifically but said all Russian government internet resources were “one way or another… under constant cyberattacks”, including the presidential site, which is targeted “round the clock”.
He said the proliferation of AI technology was contributing to an increase in cyberattacks.
Herbert Lin, a senior research scholar for cyber policy and security at Stanford University, told BBC World Service’s Newsday programme: “I think it’s the first time it’s happened to an official Ukrainian information app, but I suspect it won’t be the last.”
Oleksander Merezhko, chair of the foreign affairs committee in the Ukrainian parliament, told the BBC the attack did not cause panic or extreme reaction.
Merezhko said Ukrainians, especially in Kyiv, were used to “practically non-stop alerts”, adding “this message was in Russian language which immediately aroused suspicions”.
He continued: “It’s a very unpleasant fact but it is part of cyber war and people in Ukraine understand that. It is possible and we are quite prepared for such possibilities.”
The alerts in Ukraine were sent minutes before the end of a localised ceasefire in Ukraine’s front-line Kherson region, which was implemented last Tuesday.
Fighting continued elsewhere while the Kherson ceasefire was in place, which was brokered in part by US First Lady Melania Trump to allow civilians to evacuate the Russian-occupied city of Oleshky.
Russia launched a full-scale invasion of Ukraine in February 2022.