EU foreign ministers on Friday 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.
"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.
Another European foreign minister, asking not to be identified in order to speak more freely, told AFP: "Our conviction is that there is a risk of a Russian intervention in Ukraine and we must be ready to react."
When Russia annexed the Ukrainian Black Sea peninsula of Crimea in 2014 "it took us weeks to agree on our response, this can't happen again", the minister said.
Avoid escalation
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
German Foreign Minister Annalena Baerbock said there was still room for diplomacy with Russia.
"Especially in moments of crisis, diplomacy requires much stamina, patience and strong nerves," she told reporters.
Baerbock, who is to meet with Russian Foreign Minister Sergei Lavrov on Tuesday in Moscow, added: "We are doing everything to avoid any further escalation."
She said the trip was part of efforts "to use every possible channel of communication".
The United States' ambassador to NATO welcomed the EU's mobilisation for Ukraine, saying it was a valuable addition to the defence alliance's own capabilities.
"There's a real capacity now being built up in Brussels, inside the European Union, to pair with the depth of knowledge, expertise and the toolkit that NATO," Julianne Smith told reporters in Brussels.