Hackers linked to Ukraine’s main spy agency have breached computer systems at a Moscow-based internet provider in retaliation for a Russian cyber attack against Ukrainian telecom giant Kyivstar, a source with direct knowledge of the operation told Reuters on Tuesday.

The hacking group, dubbed “Blackjack”, has previously been linked to the Security Service of Ukraine (SBU). The hackers deleted 20 terrabytes of data at M9 Telecom, a small Russian internet and TV provider, leaving some Moscow residents without internet, the source said.

The digital intrusion was a warm-up for a larger cyber attack which would be “serious revenge for Kyivstar”, the source said, citing the hackers. The source did not say when the hack took place.

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