Reported by BBC World
Regular alerts warn Ukrainians to take shelter from incoming attacks but the system was compromised on Sunday
Ukraine and Russia have both said air alert apps were hacked on Sunday night after fake messages warning of attacks, including nuclear, were sent to users.
Ukraine's widely used app sent red-level air raid warnings across the country, with additional warnings of radiation, chemical attacks, and street fighting sent to some residents in areas including Kyiv.
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A few hours later, Russian authorities said a similar app was hacked after fake alerts, some of which contained foul language, were triggered.
It was unclear who was behind the cyberattacks or whether the two incidents were connected.
Many Ukrainian citizens are used to receiving almost nightly alerts warning of incoming Russian missile and drone attacks. Some told the BBC that, four and a half years into the war, they have disabled the app altogether.
After Sunday night's hack, the country's air force urged people to consult other official channels.
In a post on Telegram, the air force said: "The Air Alarm application has probably been subjected to hostile interference. There is no radiation threat! Stay calm and follow our updates on our [Telegram] channel."
The government said it had temporarily disabled the app for maintenance, saying it had "apparently been compromised".
Some of the fake warnings were written in Russian, which would have alerted many users to the fact they were not legitimate.
But Olena, a Kyiv resident, told BBC Radio 4's Today programme she had been alarmed, saying: "The first thought was 'oh my god, I'm not prepared for this'.
"I'm prepared for blackouts, for a cut in central heating and water supply, but not for nuclear attack."
Olena says her husband then received alerts for "street fighting attacks in Kyiv" and another alert, written in Russian.
"After that I understood that somebody had attacked the app, because all notifications we get in Ukrainian."
A Ukrainian government agency published translated versions of some of the fake alerts
In Russia, the government said one of its air raid alert apps had also been targeted by "a…