Ukraine's national postal service Ukrposhta said it had been hit by a cyberattack on Friday after sales of a postage stamp depicting a Ukrainian soldier making a crude gesture to a Russian warship went online.
Queues formed to buy the stamp when it went on sale at the postal headquarters in Kyiv last week following the sinking of the flagship of Russia's Black Sea fleet.
Kyiv said it had hit the cruiser Moskva with missiles. Russia said the ship sank while being towed in stormy seas after a fire caused by an explosion of ammunition.
Ihor Smilianskyi, Ukrposhta's director-general, issued an apology to customers for what he said was a DDoS (distributed denial-of-service) attack but did not say who might be behind it.
"We are really doing everything, together with internet providers, to restore both the online store and other Ukrposhta systems that are also temporarily not working due to a DDos attack on our systems," he wrote on Facebook.
Ukrainian officials have warned of the danger of cyberattacks by Russian hackers since Russia invaded the country on February 24 Moscow has consistently denied accusations it has launched cyberattacks on Ukraine.
Ukraine said earlier this month that it had thwarted an attempt by Russian hackers to damage its electricity grid with a cyberattack.