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, the European Union said on Tuesday.

The attack against Viasat's KA-SAT network in late February took place just as Russian armor 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 behavior in cyberspace, which also formed an integral part of its illegal and unjustified invasion of Ukraine," it added.

The attack 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.