"Allied experts in country are also supporting the Ukrainian authorities on the ground," Stoltenberg said, in a statement from NATO headquarters.

"In the coming days, NATO and Ukraine will sign an agreement on enhanced cyber cooperation, including Ukrainian access to NATO's malware information sharing platform," he said.

Earlier, a sweeping cyberattack had briefly hijacked Kyiv government websites, sparking speculation that Russian actors were sabotaging Ukraine, amid high stakes military tensions between the neighbours.

The European Union said it was mobilising "all its resources" to help Ukraine, and NATO and the Western allies have warned they will impose "massive consequences" on Russia if it attacks.

"NATO has worked closely with Ukraine for years to help boost its cyber defences. NATO cyber experts in Brussels have been exchanging information with their Ukrainian counterparts on the current malicious cyber activities," Stoltenberg said.

"NATO's strong political and practical support for Ukraine will continue."