Russia was behind a massive cyberattack against a satellite internet network which took thousands of modems offline at the onset of the war in Ukraine, Britain and the European Union said on Tuesday.

The digital assault against Viasat's KA-SAT network in late February took place just as Russian armour pushed into Ukraine and helped facilitate President Vladimir Putin's invasion of the country, the Council of the EU said in a statement.

"This cyberattack had a significant impact causing indiscriminate communication outages and disruptions across several public authorities, businesses and users in Ukraine, as well as affecting several EU Member States," the statement said.

"This unacceptable cyberattack is yet another example of Russia's continued pattern of irresponsible behaviour in cyberspace, which also formed an integral part of its illegal and unjustified invasion of Ukraine," it added.

The remote sabotage caused a "huge loss in communications in the very beginning of war," Ukrainian cybersecurity official Victor Zhora said in March.

Russia routinely denies it carries out offensive cyber operations. The Kremlin did not immediately respond to a request from Reuters for comment.

Western intelligence agencies, including the U.S. National Security Agency, French government cybersecurity organisation ANSSI, and Ukrainian intelligence were investigating Russia's potential role in the attack in the days after it, Reuters reported at the time.

British Foreign Secretary Liz Truss said the cyberattack was a "deliberate and malicious attack by Russia against Ukraine".

Russia's primary target was the Ukrainian military but it also disrupted wind farms and internet users in central Europe, Truss said in a statement, citing Britain's National Cyber Security Centre.

(Reporting by James Pearson; Additional reporting by William James in London; Editing by Jan Harvey, William Maclean and Angus MacSwan)