Ukrtelecom, Ukraine’s telephone and internet company, suffered a large cyberattack, the State Special Communications Service of Ukraine reported on Monday. The agency said Russian forces were responsible.

Ukrtelecom is the biggest internet provider in the country in terms of geographical coverage, and second after Kyivstar in terms of the number of clients.

The provider reduced service to most private users and business clients during the attack in order to be able to continue providing it to Ukraine’s military, the technical security and intelligence service of Ukraine, said on social media.

NetBlocks, an independent global internet monitor, reported it was the most serious attack on the provider since Russia invaded Ukraine last month, reducing Ukrtelecom connectivity to only 13 percent of prewar levels.

It took several hours before the company could stop the attack and restore normal services to all customers, the communications agency reported.