MOSCOW>> Russian state media company VGTRK, which owns and operates the country’s main national TV stations, was targeted in a massive cyberattack on Monday that a Ukrainian government source said Kyiv’s hackers had caused.

The website of VGTRK, the All-Russia State Television and Radio Broadcasting Company, was not loading early on Monday and its Rossiya-24 rolling 24-hour news channel was not available online.

“503 Service Unavailable. No server is available to handle this request,” read an error message when Reuters reporters tried to access the livestream.

“Our state media holding, one of the largest, has faced an unprecedented hacker attack on its digital infrastructure,” Kremlin spokesman Dmitry Peskov told reporters, saying VGTRK was working to overcome the consequences.

“Specialists are working to find out all the circumstances, to understand where the traces left behind by those who organized this hacker attack on the critical infrastructure object lead.”

VGTRK, which said earlier on Monday that its online service had come under cyberattack overnight and whose news channels provide many Russians with news on the war in Ukraine, did not immediately respond to a Reuters request for comment.

A Ukrainian government source said Ukrainian hackers were responsible for the incident, which coincided with Russian President Vladimir Putin’s 72nd birthday.

“Ukrainian hackers ‘congratulated’ Putin on his birthday by carrying out a large-scale attack on the all-Russian state television and radio broadcasting company,” the source told Reuters, asking not to be named.

Reuters could not independently verify that assertion.

Russian news outlet Gazeta.ru cited an unnamed source as saying that the cyberattack had targeted the online and internal services of VGTRK, which also owns and operates radio stations and many regional TV channels.

“Online broadcasting and internal services are down and even the Internet and telephony are not working. It’s going to take a long time to fix,” it quoted the source as saying.

“I heard they (the hackers) wiped everything from the servers, including backups. They’ve been working in lockdown since six in the morning.”

Speaking in Moscow, Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, did not say who was behind the attack but told reporters that Russian media had long since become targets for what she called “the collective West” and said what had happened was part of “a hybrid war”.

Moscow would raise the cyberattack in all international forums, including at UNESCO, the U.N. agency that promotes freedom of speech, Zakharova said.