Ukraine's postal service Ukrposhta was hit by a cyberattack Friday after sales of a postage stamp depicting a Ukrainian soldier making a crude gesture to a Russian warship went online, the national postal service said.

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 Feb. 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.

Ukraine had claimed the Moskva's damage was the result of one of its missile strikes, while Russia's Defense Ministry had spoken only of a fire breaking out and of exploding ammunition. Although Russia did not acknowledge that Ukrainian missiles had hit the ship, early on Friday it struck what it described as a factory in Kyiv that made and repaired anti-ship missiles, in apparent retaliation. The Pentagon backed Ukraine's claim that the Moskva warship was hit by two missiles before it sank in the Black Sea.

The Moskva was by far Russia's largest vessel in the Black Sea fleet, equipped with guided missiles to attack the shore and shoot down planes, and radar to provide air defense cover for the fleet.