Late Sunday, users of Ukraine's 'Air Alert' app received false warnings of radiation hazard, chemical threat and street combat, with one message in Russian declaring all of Ukraine under nuclear alert and adding, in a casual tone, that 'Oreshnik is in action,' a reference to Russia's nuclear-capable hypersonic missile.

Ukraine's Air Force said on Telegram that the app had 'likely been compromised by the enemy' and that there was no radiation threat. Officials on both sides said their national alert systems were operating normally afterward.

Al Jazeera reports this appears to be the first time either country's official alert apps have been hacked, even though cyberwarfare has intensified since Russia's full-scale invasion in 2022, from the Sandworm attack that took Kyivstar offline to Ukraine's volunteer IT Army targeting Russian infrastructure.

BBC's cyber correspondent writes the alerts had 'all the hallmarks of hacktivism' — colloquial, even jokey wording rather than official military messaging — and notes one fake message contained a link to a newly created Telegram channel run by an anti-Ukrainian group that quickly gained 15,600 subscribers and posted propaganda claiming Ukraine is 'losing decisively.'

The BBC also reports that a Ukrainian hacktivist community contact called the strike on Russia's system 'most likely a retaliatory hack,' saying alert systems had not been targeted before 'due to moral and ethical considerations.'

Neither government named suspects, and Ukrainian MP Oleksandr Merezhko said Kyiv would neither confirm nor deny responsibility for the Russian hack.

Millions of Ukrainians rely on the Air Alert app to seek shelter before incoming missiles and drones strike, so any erosion of trust carries real danger. EU foreign policy chief Kaja Kallas told the BBC that Russia is trying to 'hurt the civilians as much as possible' so that alerts stop working.

The nuclear-radiation theme is especially raw in Ukraine: Al Jazeera notes the country carries the memory of the 1986 Chornobyl disaster and ongoing fighting near the Zaporizhzhia nuclear plant, Europe's largest, where the UN nuclear watchdog said last week a welder was killed in a drone attack.

Encouragingly, many Ukrainians responded with memes rather than panic, and officials urged people to trust sirens and loudspeakers over app messages.

No group has claimed responsibility and it remains unclear whether the two incidents were connected, though the near-simultaneous timing of attacks on two separate alert systems suggests a possible retaliation cycle.

The UK said it does not yet know who was behind the Ukrainian hack but is monitoring closely, with Defence Secretary Wes Streeting citing Europe's record of 'hybrid or state-sponsored terrorist attacks driven by Moscow,' according to the BBC.

Meanwhile, expectations have shifted back to war: Al Jazeera reports many Ukrainians expect renewed fighting along the front lines after the ceasefire's end, and Ukraine's Armed Forces accused Russia of using the truce to rotate units and fortify positions — which Kremlin spokesman Dmitry Peskov called 'completely common practice.'