By Guy Faulconbridge

LONDON (Reuters) -Russia's Federal Security Service (FSB) said on Thursday it had uncovered a U.S. National Security Agency (NSA) plot using previously unknown malware to access specially made so-called backdoor vulnerabilities in Apple phones.

The FSB, the main successor to the Soviet-era KGB, said that several thousand Apple phones had been infected, including those of domestic Russian subscribers.

The Russian spy agency also said telephones belonging to foreign diplomats based in Russia and the former Soviet Union, including those from NATO members, Israel, Syria and China, had been targetted.

"The FSB has uncovered an intelligence action of the American special services using Apple mobile devices," the FSB said in a statement.

Neither Apple nor the NSA immediately responded to emailed requests for comment outside usual U.S. business hours.

'SOFTWARE VULNERABILITIES'

The FSB said the plot showed the close relationship between Apple and the NSA, the U.S. agency responsible for U.S. cryptographic and communications intelligence and security.

"The hidden data collection was carried out through software vulnerabilities in U.S.-made mobile phones," Russia's foreign ministry said in a statement.

"The U.S. intelligence services have been using IT corporations for decades in order to collect large-scale data of Internet users without their knowledge," the ministry said.

The United States is the world's top cyber power in terms of intent and capability, according to Harvard University's Belfer Center Cyber 2022 Power Index, followed by China, Russia, the United Kingdom and Australia.

Shortly after Russia sent its troops into Ukraine last year, U.S. and British spies claimed a scoop by uncovering - and going public with - intelligence that President Vladimir Putin was planning to invade.

It is still unclear how that intelligence was gained.

Officials in Russia, which Western spies says has constructed a very sophisticated domestic surveillance structure, have long questioned the security of U.S. technology.

Putin has always said he has no smartphone, though the Kremlin has said the former KGB spy does use the internet from time to time.

Earlier this year, the Kremlin told officials involved in preparations for Russia's 2024 presidential election to stop using Apple iPhones because of concerns that the devices are vulnerable to Western intelligence agencies, the Kommersant newspaper reported.

At a Kremlin-organised seminar for officials involved in domestic politics, Sergei Kiriyenko, first deputy head of the presidential administration, told officials to change their phones by April 1, Kommersant said, citing unidentified sources.

(Writing by Guy FaulconbridgeEditing by Gareth Jones and Mark Potter)