Earlier this week, Ukrainian mobile operator Kyivstar was hit by the biggest cyberattack of the war so far.

The outage, which first hit the telco on Tuesday (December 12), left millions without mobile services.

Kyivstar has more than 24 million mobile customers, and more than 1.1 million home Internet customers.

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The cyberattack impacted IT infrastructure and air raid alert systems in some of the regions.

Kyivstar chief executive officer Oleksandr Komarov blamed the attack on Russia.

"War is also happening in cyberspace. Unfortunately, we have been hit as a result of this war," he told national television.

"[The attack] significantly damaged [our] infrastructure, limited access, we could not counter it at the virtual level, so we shut down Kyivstar physically to limit the enemy's access."

Following the attack on Tuesday, Russian hacktivist group Killnet claimed responsibility for the attack via a statement on the Telegram messaging app, but did not provide evidence.

In a statement yesterday, Kyivstar called the attack "unprecedented," and said service will be restored gradually, warning that further network outages are still possible.

"Currently, we are still restoring mobile Internet and SMS — we hope that these services will also work within the day," the company said yesterday.

"After the full stabilization of the network, we will definitely provide compensation to all subscribers and corporate clients who were not connected and could not use our services."

Doug Madory, director of Internet analysis at networking firm Kentik, noted that the operator is "still only passing a fraction of the traffic is normally handles following a reported cyberattack against the company."

In March of last year, just weeks after Russia invaded Ukraine, national telecoms operator Ukrtelecom was hit by a 15-hour outage, where connectivity collapsed to 13 percent of pre-war levels.

The government blamed the outage on a cyberattack.

Other outages have hit the country since, including from physical attacks on infrastructure.