Ukraine’s military intelligence agency, the GUR, launched a cyberattack against the online services of Russia's ruling United Russia party late last week, according to a source in the spy agency who spoke to several Ukrainian media outlets.

The attack targeted United Russia’s servers, websites and domains, rendering the party’s digital platforms "partially inaccessible." The agency didn’t provide any further details about the operation.

United Russia said on Friday on its official Telegram channel that it experienced "massive" distributed denial-of-service (DDoS) attacks on all of its services but assured that the party's critical digital infrastructure remained operational.

It’s rare for a Kremlin-aligned organization to admit such an incident. DDoS attacks involve floods of internet traffic intended to overwhelm websites. 

The incident occurred on the day when Russia hosted its “Victory Dictation” — a patriotic online initiative in which people throughout Russia and in 60 countries worldwide take a test about Russian history. Russian state officials said the timing indicated that Russia’s enemies aimed to “undermine all efforts to preserve historical truth.”

“In this way, they are trying to prevent millions of people from expressing respect for the memory of the history of the Great Patriotic War,” or World War II, United Russia said in a statement.

The head of Russia’s telecom giant Rostelecom, Mikhail Oseevsky, said that the recent attack was more powerful than the one that targeted the party’s website on the day of the presidential election earlier in March. This time there were 1.5 million fake requests per second, he said.

Over the weekend, Ukraine’s military intelligence also announced that, in cooperation with the Ukrainian hacker group BO Team, it launched a cyberattack on a subsidiary of one of Russia’s major telecom providers, MTS.

BO Team told Ukrainian media that they destroyed the company’s software and configuration files, causing severe disruptions to internet services across Russia, including in Moscow and St. Petersburg.

As with many cyberattacks claimed by the GUR, verifying the validity of this one is challenging due to the lack of information from the Russian side.