Kyivstar has 24.3 million mobile subscribers - more than half of Ukraine's population - and over 1.1 million home internet subscribers. Its CEO Oleksandr Komarov said the attack was "a result of" the war with Russia.

He did not say which Russian body he believed to be responsible, and that the company's IT infrastructure had been "partially destroyed."

"The attacksignificantly 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," Komarov said.

A source close to Ukraine's cyber defence also said that Russia was suspected to be the source of the attack, but no specific group had been identified. "It's a state actor," said the source, who asked not to be identified because of the delicacy of the issue, adding that data cable interception showed "a lot of Russian controlled traffic directed at these networks."

 "There's no ransom. It's all destruction. So it's not a financially motivated attack," said the source. Ukrainian officials said that the cyber-attack affected air raid alert systems in more than 75 settlements in the central Kyiv region.

Komarov said two customer data databases had been damaged and were currently locked. "The most important thing is that the personal data of users has not been compromised," Kyivstar said in its statement, promising to compensate customers for losing access to services.

Meanwhile, Ukraine's defence intelligence director (GUR) said it infected thousands of servers belonging to Russia's state tax service with malware and destroyed databases and backups.

According to GUR's statement published Tuesday, the attack led to the 'destruction' of the agency's infrastructure.

GUR claimed they destroyed configuration files 'which for years ensured the functioning of Russia's tax system.'