The announcement from Ukraine's communications watchdog comes with the former Soviet republic fearing a possible attack from Russian forces conducting massive military drills at its frontiers.

The affected sites include the Oschadbank state savings bank and Privat -- two of the country's largest financial institutions.

The defence ministry site showed an error message saying the site was "undergoing technical maintenance".

The armed forces website said it was inaccessible.

The watchdog said Privat and its app Privat24 were hit by "a massive denial of service (DDOS) attack".

Both banks said their online services were down.

"It cannot be excluded that the aggressor is resorting to dirty tricks," the watchdog said in reference to Russia.

Tuesday's cyberattack came one month after another strike briefly took down key government websites.

NATO responded within hours of the January attack by announcing a cyber warfare cooperation deal with Kyiv. The European Union also said it was mobilising "all its resources" to help Ukraine at the time.

The deals were designed to help protect Ukraine from Russian state actors and private proxies committing cyber crimes on the Kremlin's behalf.

Kyiv said the damage in January had been limited and held back on apportioning blame.

Tuesday's attack came the same day that Russia announced it was pulling back some of the troops deployed on Ukraine's borders in Moscow's standoff with the West over NATO's presence in eastern Europe.