Russia has dismantled ransomware crime group REvil at the request of the United States in an operation in which it detained and charged the group's members, the FSB domestic intelligence service says.
The arrests were a rare apparent demonstration of collaboration between Russia and the United States at a time of high tensions between the two over Ukraine.
The announcement came as Ukraine was responding to a massive cyber attack that shut down government websites although there was no indication the incidents were related.
A police and FSB operation searched 25 addresses, detaining 14 people, the FSB said, listing assets it had seized including 426 million roubles ($A7.7 million), $US600,000, 500,000 euros, computer equipment and 20 luxury cars.
A Moscow court identified two of the men as Roman Muromsky and Andrei Bessonov and remanded them in custody for two months.
Muromsky could not be reached for comment and his phone was off.
Reuters could not immediately reach Bessonov.
Two Muscovites told Reuters Muromsky was a web developer who had helped them with websites for their businesses.
Russia told the US directly of the moves it had taken against the group, the FSB said.
The US embassy in Moscow said it could not immediately comment.
"The investigative measures were based on a request from the... United States," the FSB said.
"The organised criminal association has ceased to exist and the information infrastructure used for criminal purposes was neutralised."
The REN TV channel aired footage of agents raiding homes and arresting people, pinning them to the floor and seizing large piles of US dollars and Russian roubles.
The group members have been charged and could face up to seven years in prison, the FSB said.
A source familiar with the case told Interfax that the group's members with Russian citizenship would not be handed over to the US.
The US government said in November it was offering a reward of up to $US10 million ($A14 million) for information leading to the identification or location of anyone holding a key position in the REvil group.
The US has been hit by a string of high-profile hacks by ransom-seeking cybercriminals.
A source with direct knowledge of the matter told Reuters in June that REvil was suspected of being the group behind a ransomware attack on the world's biggest meat packing company, JBS SA.
The US government has repeatedly accused the Russian state in the past of malicious activity on the internet, which officials in Moscow deny.
REvil has not been associated with any major attacks for months.
John Shier, a threat researcher at the UK-based Sophos cybersecurity company, said there was no independent confirmation the self-identified leaders of the "defunct" group had been arrested.
"If nothing else, it serves as a warning to other criminals that operating out of Russia might not be the safe harbour they thought it was," he said.
A former client of Muromsky who only gave the name Sergei described him as a regular worker who did not appear wealthy.
Sergei runs a shop called Motohansa selling motorcycle spare parts.
Muromsky created its website and supported it for some time charging him around 15,000 roubles per month, he said.
"He is a smart person and I can imagine that if he wanted to do it (hacking) he could, but he charged very little money for his services. Several years ago he had a Rover car. That's not an expensive car at all," Sergei said.
Muromsky is in his 30s and was born in Anapa in Russia's south, he said.
"He worked as a normal programmer."
Another client, Adam Guzuyev, described Muromsky as "a regular normal worker" who proved unable to install all the features Guzuyev wanted on his website.
"He earned no more than 60,000 roubles. I can't say he has genius abilities," he said, adding Muromsky spent three months working on his website.
Australian Associated Press