(Reuters) — U.S. and British officials on Thursday accused the Russian government of running a years-long campaign to hack into critical infrastructure, including a nuclear plant in Kansas and a Saudi oil refinery.

The announcement was paired with the unsealing of criminal charges against four Russian government officials, whom the U.S. Department of Justice accused of carrying out two major hacking operations aimed at the global energy sector.

Britain's Foreign Office said that the hackers targeted the systems controlling the Wolf Creek nuclear plant near Burlington, Kansas, “but failed to have any negative impact.”

Thousands of computers in 135 countries were affected between 2012 and 2018 by the hacking efforts, U.S. prosecutors said.

Cybersecurity analysts described the moves as a shot across the bow to Moscow after U.S. President Joe Biden warned just days ago about “evolving intelligence” that the Russian government may be preparing cyberattacks against American targets.

John Hultquist, whose company Mandiant investigated the Saudi refinery hack, said that by making the criminal charges public the United States has “let them know that we know who they are.”

In one of the two indictments unsealed Thursday and dated June 2021, the Justice Department accused Evgeny Viktorovich Gladkikh, a Russian ministry of defense research institute employee, of conspiring with others between May and September 2017 to hack the systems of a foreign refinery and install malware known as “Triton” on a safety system produced by Schneider Electric.

The refinery wasn't named, but the British government said it was in Saudi Arabia and it has previously been identified as the Petro Rabigh refinery complex on the Red Sea coast.

In a second indictment, dated August 2021, the Justice Department said three other suspected hackers from Russia's Federal Security Service carried out cyberattacks on the computer networks of oil and gas companies, nuclear power plants, and utility and power transmission companies between 2012 and 2017 — a campaign researchers have long attributed to a group sometimes dubbed “Energetic Bear” or “Berserk Bear.”

The Russian Embassy in Washington did not immediately return a message seeking comment.

“Russia's targeting of critical national infrastructure is calculated and dangerous,” U.K. foreign secretary Liz Truss said in a statement. She said it showed Russian President Vladimir Putin is prepared to risk lives to sow division and confusion among allies.

A Justice Department official told reporters that even though the hacking at issue in the two cases occurred years ago, investigators remained concerned Russia will carry out similar attacks in the future.

“These charges show the dark art of the possible when it comes to critical infrastructure,” the official said.