Sweden spied on Russian officials and passed their findings along to the United States, according to leaked documents obtained by Swedish Television.
The documents come from American whistleblower Edward Snowden and reveal the key role played by Sweden's National Defence Radio Establishment (FRA) in the American spy apparatus.
It is unclear how the spying was carried out, whether the FRA - the Swedish national authority for signals intelligence - tapped telephone lines or accessed internet data, and exactly who was targeted by the intelligence services.
The US National Security Agency (NSA), however, said it was very pleased with Sweden's cooperation, calling the work "unique". Ahead of a meeting with FRA officials, NSA managers were told to praise the Swedes for their assistance.
"Thank Sweden for its continued work on the Russian target, and underscore the primary role that FRA plays as a leading partner to work the Russian target, including Russian leadership…and…counterintelligence," one document read.
Swedish Television's investigative news program Uppdrag Granskning obtained the documents and will air their report on December 11th. Producers of the program worked alongside American journalist Glenn Greenwald, who has written extensively about secret NSA documents and Snowden.
Officials with both the FRA and NSA declined to comment on the report.
Speaking with news agency TT, Defence Minister Karin Enström said Sweden needs to take necessary measures to protect itself and that clear laws govern intelligence activities.
"We need to conduct intelligence operations to protect Sweden against external threats." said Enström. "We have an operation that takes place within the framework of the FRA, with clear legislation, strict control and parliamentary oversight. But how it is done, and with whom Sweden cooperates, is not public information," she added.