Hackers broke into emergency alert apps in Ukraine and Russia late on Sunday night.
The Air Raid Alert app warned of a radiation threat in Kyiv at about 11 PM on Sunday.
The messages mentioned the Oreshnik missile, chemical attacks, and street fighting.
The alerts mixed Russian and Ukrainian words and had spelling errors and swear words.
Ukraine's air force said on Telegram: there is no radiation threat, stay calm.
Early on Monday, Russia's Emergencies Ministry said its own app suffered a massive cyberattack.
One false message claimed nuclear warheads had been launched across all regions of Russia.
Russia said its sirens, SMS alerts, and loudspeakers kept working during the attack.
Many users laughed it off. One Kyiv resident wrote: Good thing our mayor is a boxer.
The hacks came right after a six-day ceasefire in the Kherson region ended on Sunday.
Ukraine's app was restored overnight. Russia says fixes are underway. Both are tightening security.
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