What happened?Phone notifications were sent to millions across Ukraine late Sunday warning of a "nuclear alert", a "chemical threat" and even "street fights" in Kyiv. One of the messages was in Russian and included a link to a new Telegram account named "Hitcore", the logo of which is a metallic eagle emblazoned over the Russian tricolour flag.A few hours later, Russia's emergency situations ministry said that a "massive cyberattack" had hit its own application, with "false information" sent to the public. Several regional social media accounts run by the department sent out messages warning of a imminent nuclear attack on Russia. For instance, a Telegram message was sent by authorities in the Belgorod border region warning "nuclear warheads ... have been launched at all regions of Russia", and telling people to "urgently stock up on food for three weeks".Which apps were affected?Ukraine's air alert app is widely used to warn of Russian missile and drone attacks. It has been downloaded more than 46 million times and has more than 6.7 million daily users, according to developers, Ajax Systems.Russia does not have a nationwide air alert system, and beyond border regions does not routinely send warnings to citizens of incoming Ukrainian attacks. Few Russians use the affected application, run by the ministry of emergency situations.Act of war?It remains unclear if the attacks were part of the war, carried out by either side, or launched by independent actors or groups. Ukraine's Centre for Strategic Communication, a government-backed project set up to counter Russian propaganda and online threats, said "Russians have hacked" the app. A pro-Russian group calling itself Hitcore claimed responsibility for the hack of the Ukrainian alert system.Nobody has claimed responsibility for the hack of the Russian system.