Kyivstar, the primary mobile network in Ukraine, has declared itself a victim of a "significant cyber attack," leaving customers without phone and internet services, and causing air raid sirens in one city to malfunction.

The CEO of Kyivstar suggested a potential connection to Russia, while Ukraine's security services are actively investigating the incident. The outage has impacted around 24 million mobile users and one million home internet users on the Kyivstar network.

Reports indicate disruptions in mobile and internet signals for individuals and businesses and malfunctioning air raid sirens in Sumy.

PrivatBank, Ukraine's largest bank, also reported issues with some cash machines.

The Security Service of Ukraine (SBU) has initiated an investigation into the cyber attack. Moscow has not issued a statement regarding the incident.

"One of the versions currently being investigated by SBU investigators is that the Russian special services may be behind this hacker attack," SBU said in a statement.

Kyivstar said earlier it had called in law enforcement agencies because of the "illegal interference".

The company's general director, Oleksandr Komarov, also appeared to suggest Russian actors could be to blame.

"The war with Russia has many dimensions and one of them is in cyberspace," he said.

"We are working to restore communication as soon as possible," he added and promised compensation for affected customers.

The operator said no-one's personal data had been compromised.

Russia launched a full-scale invasion of Ukraine in February 2022, with the war now well into its second year.

Dutch-domiciled parent company Veon said it would work with Kyivstar on "additional security measures" to prevent similar incidents in the future.

Separately, Ukraine's military intelligence claimed to have carried out a cyber-attack on Russia's federal tax system in recent days.

"During the special operation, military intelligence of Ukraine managed to break into one of the well-protected key central servers of the federal tax service," read a statement on Telegram.

It said Russian efforts to restore services were now on their fourth day.

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