The CEO of Ukraine’s largest telecommunications company said Tuesday that a Russian cyberattack took down its service, severing internet access for millions across the country.

Service at the company, Kyivstar, “collapsed” Tuesday, according to NetBlocks, a company that monitors global internet outages. Both phone and internet access are down, Kyivstar said on its Facebook page.

The outage appears to be one of the largest cyberattacks against a civilian communications system to date. At the start of Russia’s invasion of Ukraine, hackers that analysts traced to Russia launched an attack on Viasat, a U.S. satellite internet company popular in Ukraine.

Russia’s Ministry of Foreign Affairs didn’t immediately respond to a request for comment.

Kyivstar CEO Oleksandr Komarov said in a TV appearance that Russia was responsible for the outage.

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“The war with the Russian Federation has many dimensions, and one of them is in cyberspace,”  he said. “Unfortunately, this morning the operator became the target of a superpowerful cyberattack, because of which communications services and internet access are unavailable.”

“The period for restoring services is not completely clear. We are working to eliminate the consequences of this attack to resume communication as quickly as possible,” he said.

Kyivstar’s website was not available Tuesday, but an archived version from November said the company had more than 25 million customers, the vast majority of them mobile users.

Russia has appeared to increase its bombardment of Ukraine in recent days as Republicans have stalled a proposed aid package to the country. In response, the White House has invited Ukrainian President Volodymyr Zelenskyy to visit Washington this week.

While Russia has relied more on conventional artillery bombardment to disrupt Ukraine’s infrastructure since the war started, its hackers have also caused significant damage. Last month, Ukrainian officials admitted for the first time that an alleged cyberattack had caused a power outage in the country in 2022, though they declined to share where.