Ukraine's main mobile network, Kyivstar, says it's been the target of a "powerful hacker attack".

Axar.az reports that customers have been left without phone or internet access, while one city's air raid sirens stopped working. Kyivstar's chief executive implied Russia could be responsible.

Ukraine's security services are investigating. Moscow hasn't commented.

The Kyivstar network is estimated to have some 24 million mobile customers and a million home internet users.

Reports emerged on Tuesday morning that people and businesses had lost mobile and internet signals.

Air raid sirens in the northeastern city of Sumy also malfunctioned as a result of the outage.

Military authorities in the area announced they would send out police and emergency vehicles to alert residents of any incoming missile or drone strikes.