Ukraine pointed the finger at Russia's secret service over a massive hacker attack on Tuesday that disrupted services for the country's main phone operator.
Ukraine's SBU security service said it had opened a criminal investigation and sent agents to the company's offices.
"The special services of the Russian Federation may be behind this hacker attack," the SBU said in a statement.
Kyivstar, which has 24.3 million customers, blamed it on Russia's invasion without giving further details on the connection between the two.
"This is a war, it takes place not only on the battlefield, it also takes place in virtual space and unfortunately, we are affected as a result of this war," Kyivstar's general director Oleksandr Komarov said on national television.
"We see the main goal of this attack is the maximum possible destruction of the operator's IT infrastructure. They partially achieved this goal," he said.
"Our IT infrastructure was significantly damaged, access was limited, we could not counteract this virtually, so we physically disconnected Kyivstar from the network," he said, adding that Kyivstar was "still working" on restoring service.
The company said on Facebook it had been "the target of a powerful hacker attack."
"It caused a technical failure, resulting in mobile communications and access to the internet becoming temporarily unavailable," it said, adding consumer data had not been compromised.
The SBU said its investigation was under eight articles of the criminal code including treason, sabotage, creating malicious software and unauthorized interference in communication networks.
Ukrainian Interior Minister Ihor Klymenko reported "large-scale failures" in Kyivstar's network, leaving "many subscribers" without service.
The breakdown also affected other sectors relying on mobile roaming.
Authorities of the northeastern city of Sumy said the air raid alert system "will not work temporarily."
The blackout affected a number of other services and businesses in the country fighting off a Russian invasion for almost two years.
PrivatBank spokesman Oleg Serga said its banks and cash terminals were partly disrupted, according to public broadcaster Hromadske.
One of Ukraine's biggest banks, Monobank, said it had been targeted by a "massive DDoS attack," its co-founder Oleg Gorokhovsky said on Telegram.
Ukraine's military intelligence service GUR also on Tuesday said it had hacked into the Russian tax system, calling it a "successful special operation."
