Ukraine and Russia each say their public emergency alert apps were hacked to send false warnings, the BBC, the Kyiv Independent and ABC News of Australia report.

Late on Sunday night, Ukraine's widely used Air Alert app sent red-level air raid warnings across the country. Some people in Kyiv also got false alerts of radiation, a chemical attack and street fighting, the BBC reports.

One message, written in Russian, said a nuclear alert was coming and joked that Russia's Oreshnik missile was "on its way." Oreshnik is a Russian missile that can carry nuclear warheads.

"There is no radiation threat! Stay calm," Ukraine's air force said on Telegram. Officials said there was no real danger. Regional leaders warned people not to click a link in the messages, which could be a phishing attempt.

The app's makers turned off its alerts and briefly shut it down, the Kyiv Independent reports. Sirens and other warning systems kept working. By about midnight, Ukraine's emergency service said the national alert system was working normally.

A few hours later, Russia's emergencies ministry said its app had suffered "a massive cyberattack" that sent "fake information to the public." Messages warned Russians that nuclear warheads had been launched, the Kyiv Independent reports.

A Ukrainian lawmaker said the hack did not cause panic. Ukrainians are used to "practically non-stop alerts," he said, according to the BBC.

What we know: The two apps sent false alerts, and no real attack took place.
What we don't: Who hacked Ukraine's app, and whether the two hacks were linked.