The app is used across Ukraine to warn people in real time of the threat of air strikes from Russia. ( Reuters: Alina Smut )
Both Ukraine and Russia say their public air raid alert apps have been hacked.
The app sent Ukrainians false alerts warning that some areas of the country were subject to nuclear and chemical threats.
In a separate event, the mobile app of Russia's emergencies ministry was also hit by a massive cyber attack.
Ukraine's air force says the country's public air raid alert app has been hacked.
The app for a short time showed false warnings that Kyiv and other areas were subject to nuclear and chemical alerts and the threat of street combat.
"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 announcement came minutes after the end of the truce between Moscow and Kyiv in the eastern region of Kherson.
The localised ceasefire on Tuesday, which was brokered by US first lady Melania Trump, was the longest of the war.
Ukraine's Centre for Strategic Communications (SPRAVDI) blamed Russia for the hack in a post on its X account.
Russia has intensified its attacks on Ukraine in recent weeks, with capital Kyiv coming under fire. ( Reuters: Valentyn Ogirenko )
The app is used across Ukraine to warn people in real time of the threat of air strikes from Russia.
People often have only minutes to find shelter in the case of, for example, a ballistic missile attack.
"Currently, the information broadcast (on this application) about threat levels does not correspond to reality," the Ukrainian Centre for Countering Disinformation warned in a statement.
Ukraine's digital transformation ministry said the app was not in operation, but other means of alerting the public were functioning, as were air raid sirens.
In a separate event, the mobile app of Russia's emergencies ministry was also hit by a massive cyber attack, state news agency TASS quoted the ministry.
It was not immediately clear whether the two incidents were linked.
The ministry was working to restore the app, TASS said, without providing additional details.
Russia's emergencies ministry said the application is also designed to warn people of incoming drone attacks and other threats.
US-mediated talks between Russia and Ukraine are set to resume this month, having been effectively frozen since the US and Israel launched joint strikes on Iran in February.
Ukrainian President Volodymyr Zelensky said on Sunday, local time, his country would stop hitting Russian oil refineries — as US counterpart Donald Trump has demanded — if Washington provided Kyiv with powerful Patriot interceptor missiles.
