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

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

Both resumed service later on Tuesday but the military sites remained inaccessible hours after the initial reports of the attack emerged.

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

The armed forces website showed a message saying it could not be reached.

"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.