Russia is waging a "targeted grey-zone campaign" against Europe, including airspace violations, sabotage and cyberattacks that will only escalate if the Kremlin is not challenged, the European Union's top official warns.

European Commission President Ursula von der Leyen said that in just two weeks, fighter jets had breached Estonia's airspace, while drones overflew military bases and critical infrastructure sites in Belgium, Denmark, Germany, Poland and Romania.

"Europe must respond. We must investigate every incident, and we must not shy away from attributing responsibility, because every square centimetre of our territory must be protected and safe," von der Leyen told EU politicians in Strasbourg, France.

"If we hesitate to act, the grey zone will only expand," she said on Wednesday

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