KYIV (Reuters) -Ukraine's biggest mobile network operator, Kyivstar, said it was the victim of a powerful hacker attack on Tuesday morning which has temporarily knocked out its cellular and internet signal. 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).