A "massive" hacker attack was reported on Dec. 12 by Monobank, one of Ukraine's largest banks, the company's co-founder and CEO Oleh Horokhovskyi said.
The news came as the Ukrainian telecommunications company Kyivstar said earlier in the day that it had also come under a massive cyberattack that caused widespread internet and network outages.
Horokhovskyi did not assign responsibility for the hack, but speculation immediately began that Russia was behind it.
Russia has been repeatedly accused of backing cyber-crime groups in attacks against its rivals. Moscow has also deployed its cyber capabilities against Ukraine, including attacks on government institutions, the defense sector, and energy infrastructure.
The attack on Monobank was a denial of service (DDoS) attack, in which an excessive amount of traffic is directed at a website to overload its servers, often causing disruptions to service.
Horokhovskyi said that the situation was under control.
Kyivstar is Ukraine's leading phone services provider, with over 24 million mobile customers and more than 1 million home internet customers as of September this year.
The company said its specialists are working on solving the problem with law enforcement and security services' involvement.
"The most important thing is that, as of now, the personal data of subscribers is not compromised. We sincerely apologize for the temporary inconvenience and thank you for your understanding," the company's announcement read.
"Yes, our enemies are treacherous. But we are ready to face any difficulties, overcome them, and continue to work for Ukrainians."
The company did not specify the source of the hacker attack.
Users have been reporting problems since the morning of Dec. 12, saying they are unable to make or receive calls and use internet services.
Kyivstar's website has also been down since 9 a.m. local time.