Aeroflot, Russia’s flagship airline, said it had canceled more than 50 round-trip flights on Monday, citing a failure of its information systems after a shadowy pro-Ukrainian hacking group claimed it had conducted a devastating cyberattack against the carrier.

Russian prosecutors confirmed that the airline disruption was a result of hacking and opened a criminal investigation. The Kremlin called the situation “worrying.”

Over the past months, Ukraine and groups that back it have made an effort to disrupt everyday life in Russia.

Ukrainian drones have targeted areas around Russian airports, wreaking havoc in terminals during the high vacation season. The attacks and airport closures have become near-daily occurrences in major Russian cities, causing hundreds of flight delays, cancellations and diversions, and affecting thousands of travelers.

Aeroflot said on Monday that it had to cancel dozens of flights out of Sheremetyevo International Airport, near Moscow. Most were to fly within Russia but some international ones were also canceled, the airline said, adding that it had to make “forced adjustments to the flight schedule, including partial fight cancellations.” It 56 of its 260 scheduled round-trip flights had been canceled.

The company also said that its ticket refund function was not available because the hack caused its information systems to fail. As of Monday evening, personal account access on Aeroflot’s website remained restricted.