The US has charged four Russian Government employees who allegedly targetted companies associated with the energy sector in 135 countries.

Their cyber activity is said to have taken place between 2012 and 2018.

The US Department of Justice said that hackers’ actions have caused two separate emergency shutdowns at an oil plant in Saudi Arabia.

Hackers allegedly aimed at maintaining access to the computer networks of oil and gas firms, nuclear power plants and utility and power transmission companies.

Deputy Attorney General Lisa O. Monaco said: “Russian state-sponsored hackers pose a serious and persistent threat to critical infrastructure both in the US and around the world.

“Although the criminal charges unsealed today reflect past activity, they make crystal clear the urgent ongoing need for American businesses to harden their defenses and remain vigilant.”

US Attorney Matthew M. Graves for the District of Columbia said: “We face no greater cyber threat than actors seeking to compromise critical infrastructure, offenses which could harm those working at affected plants as well as the citizens who depend on them.”

Russia has always denied allegations that it was responsible for cyber-attacks.

Last year, the international insurer Hiscox published a cyber readiness study that suggested the UK energy sector faced the highest risk of experiencing cyber incidents.