Ukraine’s biggest mobile operator Kyivstar may need several weeks to reinstate all its services after a massive cyber attack, though major services could be restored by the end of this week, the company’s CEO Oleksandr Komarov said on Thursday.

Tuesday’s attack on Kyivstar, which counts more than half of Ukraine’s population as mobile subscribers, knocked out services and damaged IT infrastructure and air raid alert systems in several regions.

Komarov said he hoped the company would be able to fully restore three major services – mobile internet, voice services, and SMS – by the end of this week.

“(To restore) all additional services, from my point of view, will probably take several weeks,” he said in televised comments.

On Wednesday, a group called Solntsepyok, believed by Ukraine’s security service SBU to be affiliated with Russian military intelligence, said in a post on the Telegram messenger that it carried out the attack. It published screenshots appearing to show that the hackers had accessed Kyivstar’s servers.

Komarov said customer data was not compromised.

“Photos appeared in Telegram channels are fabricated,” he said. Data was not damaged or lost, he stressed.

In its Telegram post revealing the hack, Solntsepyok thanked unnamed “concerned colleagues” at Kyivstar. The SBU said on Tuesday that it had opened a criminal case following the cyberattack.

Kyivstar’s CEO said the hackers used an employee’s compromised account for the attack but how exactly they got access to it is a matter of ongoing investigation.

(Reporting by Yuliia Dysa;Editing by Elaine Hardcastle Editing by Mark Potter and Elaine Hardcastle)