NATO announced Friday it planned to deepen cyber cooperation with Ukraine after a sweeping attack knocked out key government websites in Kyiv at a time of mounting tensions between Russia and the West over Ukrainian security.

"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," Secretary General Jens Stoltenberg said.

The European Union was also mobilizing to aid its close ally after the attacks temporarily brought down sites, including those of the Foreign Ministry and Cabinet.

Kyiv said the damage was limited and held back on apportioning blame but the ex-Soviet country has accused Russians with links to Moscow for previous hits on websites and key infrastructure.

Without naming Moscow, aide to Ukraine's President Volodymyr Zelenskyy, Andriy Yermak, said Western and Ukrainian intelligence believed the cyberattacks were part of a plot for the "destabilization of the situation in Ukraine."

Ukraine's Foreign Ministry described the attack that brought down its site and other government portals as "massive."

The targeted sites, including the emergencies ministry, education ministry and Cabinet, displayed a message in Ukrainian, Russian and Polish warning Ukrainians that their personal data had been compromised.

"All information about you has become public, be afraid and expect the worst," the message read.

Within hours of the breach early Friday, the SBU, the country's intelligence agency, said access to most hit sites had been restored and that the fallout was minimal.