Ukraine's Air Force said the emergency app for a short time on Sunday falsely showed Kyiv and other areas were subject to nuclear and chemical alerts as well as the threat of street combat.
An erroneous message said Russia's hypersonic Oreshnik missile had been deployed.
"The air alert app appears to have been subjected to enemy influence," the air force said on Telegram.
"There is no radiation threat! Stay calm and follow our updates on our channel!"
The abnormality later disappeared, officials said.
"It is likely that the 'Air Alert' app has been compromised by the enemy. There is no radiation threat," the air force said on Telegram.
Officials in Ukraine urged people not to click on links from unknown sources or to spread unverified information. The app is used to inform people about the beginning and end of defence alerts.
In a separate event, the mobile app of Russia's emergencies ministry was hit by "a massive cyberattack" that resulted in the spread of false information to the public, state news agency TASS quoted the ministry as saying early on Monday.
It was not immediately clear whether the two incidents were linked.
Citing the ministry, TASS said "fake information" about a supposed threat had been sent to users.
The hacks came at the end of a six-day local ceasefire at 8am on Monday AEDT, around the Russian-held town of Oleshky - the most significant truce between Moscow and Kyiv since the invasion of Ukraine nearly five years ago.
Ukraine's General Staff issued a statement saying its units, in co-operation with local authorities, "evacuated 167 civilians to safer points in the Kherson region".
It said the evacuees included 22 children and 21 people with limited mobility requiring special assistance.
The war continued in other parts of Ukraine on Sunday, with no let up in fighting along the front line and both sides kept up long-range air attacks.
Regional governors and emergency services reported one death in a strike in the Sumy region on the Russian border, a frequent target, and at least one in the central region of Vinnytsia.
Russian strikes cut electricity to hundreds of thousands of households in four Ukrainian regions overnight, while Russian authorities reported major damage to an industrial site following Ukrainian drone attacks on Nizhny Novgorod region.
"The intensity of the strikes is very high, and the Russians are seeking to maximally exploit the deficit of ballistic missile interceptors," President Volodymyr Zelenskiy said on X.
Ukraine, meanwhile, hit the Kamensky plant in Russia's Rostov region, which he said was involved in the manufacture of Russian missiles.
with Reuters and AP
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