In a Telegram post late on Sunday, Ukraine's air raid alert channel said "unauthorised parties" had interfered with the app's operation and that the service had been temporarily suspended.
It warned users not to click on links in "suspicious notifications".
Authorities in Ukraine said the app, which Ukrainians rely on to seek safety amid daily Russian missile and drone strikes, was restored overnight.
The alerts - making false claims of high radiation levels, a chemical threat and public disturbances in Kyiv - were written in English with spelling and grammatical errors as well as in a mix of Russian and Ukrainian.
One of the false posts mentioned the hacking group Hitcore.
Mash, a Russian Telegram channel with links to the security services, said it had spoken to representatives of Hitcore and that they had claimed responsibility for the attack.
That claim could not be independently verified.
Ukraine's state-run Centre for Countering Disinformation reported the breach and urged users to monitor emergency channels on social media and rely on physical alerts while the cyberattack was being addressed.
Yurii Ihnat, a spokesman for Ukraine's air force command, said the cyberattack had no impact on Ukraine's security and defence operations because the app was separate from military command systems.
Ihnat told Ukraine's private 1+1 television that the air force continued to report real-time threats through independent communication channels, including popular social media platforms.
In Russia, officials said on Monday that hackers had sent fake emergency alerts after gaining access to the Russian emergency ministry's mobile app.
The ministry did not name who carried out the attack.
In a statement, it reassured Russians that the wider public alert system was working normally, and that the mobile app acted as a supplementary service alongside alerts shared through SMS messages, media broadcasts and municipal sirens and loudspeakers.
Ukrainian authorities in the war-divided Kherson region reported multiple Russian drone attacks early on Monday, hours after a humanitarian ceasefire expired.
The regional administration said a 62-year-old woman and a 73-year-old man were hospitalised with blast injuries in the city of Kherson, across the Dnipro River from Russian-occupied territory.
The localised ceasefire ended at midnight at the start of Monday.
The brief pause in fighting was tied to a humanitarian aid mission in the Russian-occupied city of Oleshky.
The ceasefire, which began October 27, marked the most significant truce between Kyiv and Moscow since Russia's full-scale invasion of Ukraine began nearly five years ago.
The White House said US first lady Melania Trump helped broker the limited truce.
Civilian negotiators from Russia and Ukraine also helped secure the pause, which allowed besieged residents of Oleshky to be evacuated in a rare joint operation that ended on Saturday.
An estimated 2000 civilians were in Oleshky when the temporary truce took effect.
They could leave to territory controlled by either Ukraine or Russia, or remain in the city with replenished food and medical supplies.
Exact evacuation totals were withheld to protect those who stayed.
Ukraine's general staff said Russia's military had bolstered its drone defences and replenished ammunition supplies in the area during the ceasefire.
