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CloseUkraine's emergency alert app issued a nationwide air raid warning, as well as alerts for a radiological threat, a chemical attack and street fighting in Kyiv on Sunday evening - all of which turned out to false.
A few hours later, Russia said its equivalent app had also issued false alerts, with both countries claiming their systems had been hacked.
It is still unclear who is to blame for the cyber-attacks and whether the two incidents are connected - the BBC's cyber correspondent Joe Tidy explains what we know so far.
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