The European Union is mobilising to help Ukraine deal with the cyberattack, EU foreign policy chief Josep Borrell says.
EU foreign ministers have warned Russia of a "robust" response, fearing that a cyberattack against Ukraine could prepare the ground for military action by Moscow against its neighbour.
Even before Friday's assault on key Ukrainian government websites, European ministers had warned that cyberattacks could precede, or accompany, a military incursion that Russia may be planning as it masses 100,000 troops on the Ukraine border.
The standoff with Russia "is serious, more serious than anything we've seen in recent years", Austrian Foreign Minister Alexander Schallenberg told reporters at a meeting of the bloc's top diplomats in the French city of Brest on Friday.
"Some say the cyberattack could be the prelude for other activities, military activities," he said.
Swedish Foreign Minister Ann Linde said, "This is exactly the kind of thing that we have warned of and that we are afraid of,"
"If there are attacks against Ukraine, we will be very harsh and very strong and robust in our response," Linde said.
Help to Ukraine
The EU was meanwhile mobilising to aid Ukraine deal with the cyberattack, EU foreign policy chief Josep Borrell said.
Borrell said the EU's political and security committee would convene later on Friday for an urgent meeting on the attack and the EU's rapid response cyber unit was also being activated.
"We are mobilising all our resources to help Ukraine deal with this type of cyberattack," Borrell said.
Ministers left little doubt that they suspected Russia was behind the cyberattack.
"You can imagine who did this," Borrell said, acknowledging however that "we don't have proof".
Ministers said sanctions against Russia were among options on the table to persuade President Vladimir Putin that invading Ukraine would have severe consequences for Moscow.
"Our aim is to deter Russia," said French Foreign Minister Jean-Yves Le Drian, whose country currently holds the rotating EU presidency.
"We have a joint determination to act," he said.
Meanwhile, Ukraine has proposed three-way talks with US President Joe Biden and Russian President Vladimir Putin.
"President Zelenskyy proposed to President Biden and we think it can be worked (out) to organise a trilateral meeting, maybe by videoconference, between President Biden, President Zelenskyy and President Putin," said the head of the Ukrainian president's office, Andriy Yermak.
"We are still waiting for the reaction on this, I think, from the Russian side. But our American partners take our proposal with some interest," he told the Atlantic Council think tank in Washington.