Authorities in Ukraine and Russia said their emergency-alert apps had been targeted by cyberattacks, with users in Ukraine receiving false warnings of an imminent nuclear threat.

It was unclear the attacks late Sunday were related. Ukraine said its service, which residents rely on to seek safety amid daily Russian missile and drone strikes, was up and running again after being suspended because of the attack.

Russian authorities said Monday that hackers sent fake emergency alerts after gaining access to the Russian Emergency Situations Ministry's mobile app, and that efforts to restore the app's functionality were ongoing.