Ukraine’s biggest mobile network operator, Kyivstar, said it was the victim of a powerful hacker attack on Tuesday morning which has temporarily knockedout its cellular and internet signal.

The company, which is owned by Amsterdam-listed mobile telecoms operator Veon, said it was working to repair the outage and was cooperating with law enforcement bodies.

Separately, the co-founder of Monobank, a major Ukrainian payment system, said in a social media post that his company was currently suffering a distributed denial of service (DDoS) attack, but that everything was “under control.”Reuters correspondents in Kyiv suffered disruption to their Kyivstar cellular signal throughout the day on Tuesday -- the signal was still absent at 1315 local time (1115 GMT).

“The most important thing is that the personal data of users has not been compromised,” Kyivstar said in its statement, promising to compensate customers for loss of access to services.

Kyivstar did not immediately say who it believed to be responsible.

However, Ukrainian state bodies and companies have often accused Russia, which is at war with Ukraine, of orchestrating cyber attacks against them in the past.