Speaking in Strasbourg, EU Foreign Policy Chief Josep Borrell said that he believed cyber attacks, like one Ukraine suffered last week, could be carried out.
Western countries say they fear Russia is preparing a pretext for a new assault on Ukraine, which it invaded in 2014.
Moscow denies any plans for an attack, but has said it could take unspecified military action unless the West agrees to a list of demands, including banning Ukraine from ever joining NATO.
Talks last week ended with no breakthrough. Kyiv has asked Western countries for arms to help it protect itself.