NATO on Monday inked a deal to bolster its cyber support for Ukraine, after a sweeping hacking attack against Kyiv heightened tensions amid fears that Russia could be plotting an invasion. 

Ukraine said on Sunday that it had evidence Moscow was behind the attack last week that knocked out government websites -- but the Kremlin has denied any involvement. 

NATO Secretary General Jens Stoltenberg said Friday that experts from NATO and its members were already on the ground, working with Ukraine to tackle the latest cyber attack. 

He said the new agreement would involve "enhanced cyber cooperation, including Ukrainian access to NATO’s malware information sharing platform."

"We have successfully worked with Ukraine for several years, delivering key capabilities and exchanging knowledge," Ludwig Decamps, the head of the NATO Communications and Information Agency, said on Monday. 

"Under this renewed agreement, we will deepen our collaboration with Ukraine to support them in modernising their information technology and communications services, while identifying areas where training may be required for their personnel."

Ukraine's ambassador to NATO, Natalia Galibarenko, said "with NATO's support we plan to further introduce modern information technologies and services into the command and control system of the Armed Forces of Ukraine".

The cyberattack on Ukraine last week came as tensions have soared between Moscow and the West over allegations the Kremlin could be plotting a fresh incursion into its neighbour after massing some 100,000 troops at the border. 

Western allies have warned that any land invasion could be presaged by a hacking attack designed to knock out key infrastructure in Ukraine. 

US software giant Microsoft warned Sunday that the cyberattack last week could prove destructive and affect more organisations than initially feared.

Microsoft said it was continuing to analyse the malware and warned it could render government digital infrastructure inoperable.

Moscow and NATO allies failed to make a breakthrough to ease tensions over Ukraine at high-stakes talks last week. 

The Kremlin has laid down a series of demands to NATO and the United States, including ruling out granting Ukraine membership in the alliance.