America's National Security Agency helped Israel code the Stuxnet computer worm used to attack Iranian nuclear facilities several years ago, according to former NSA contractor and whistle-blower Edward Snowden.
Involvement of the US and Israel in creation and deployment of the sophisticated malware has been suspected for some time. But the NSA's role has remained largely unknown.
Snowden made the claim in an interview with WikiLeaks associate Jacob Appelbaum in May and published by the German weekly Der Spiegel on Sunday.
"The NSA and Israel wrote Stuxnet together," Snowden said.
Stuxnet came to public attention in 2010 when it was reported to have knocked out about a fifth of the centrifuges Iran was using to enrich uranium.
From the outset, computer security experts argued the worm's unprecedented level of sophistication indicated it was a government-led cyberattack.
The New York Times chief Washington correspondent David Sanger reported the US and Israeli governments were behind Stuxnet. Drawing on his earlier reports for the Times, Sanger wrote in his 2012 book Confront and Conceal that Israel's Unit 8200 and the NSA took a key role in designing the worm.
Snowden's claims back up Sanger's anonymous White House sources and place the NSA at the heart of the operation. In light of the new information, Washington's response to Snowden may - once again - be one of contradictions.
"On one hand, they'll claim that there is nothing new to debate, but on the other hand, the US may claim that irrevocable damage has been done to national security," said Ross Anderson, professor of security engineering at Cambridge University's Computer Laboratory.
Despite the Stuxnet offensive, Anderson said cyberwarfare was still unlikely to replace military force any time soon.
"It takes months and years of preparations to create these viruses and they have to be crafted individually for attacking each target," he said, adding that it would take six separate programmes just to cut off the electricity in Britain. "Planes, tanks and ships, however, can be manufactured and sent to anywhere in the world."