Russian hackers were inside Ukrainian telecoms giant Kyivstar's system from at least May last year in a cyberattack that should serve as a "big warning" to the West, Ukraine's cyber spy chief told Reuters.

The hack, one of the most dramatic since Russia's full-scale invasion nearly two years ago, knocked out services provided by Ukraine's biggest telecoms operator for some 24 million users for days from Dec. 12.

Hackers tied to Russian military take responsibility for hack on Ukraine's biggest provider.

Ukraine's biggest mobile network operator was hit on Tuesday by what appeared to be the largest cyberattack of the war with Russia so far, knocking out mobile and internet services for millions and the air raid alert system in parts of Kyiv region.