Information collected about Russian politicians by Sweden's main signals intelligence agency, the National Radio Defence Establishment (Försvarets radioanstalt, FRA), was handed over to the US spy agency, according to documents reviewed by Sveriges Television (SVT) investigative news programme Uppdrag gränskning (UG).
The documents describe FRA as a "leading partner" in the NSA's international cooperation to monitor communications traffic around the world.
"The FRA provided NSA (…) unique collection on high-priority Russian targets, such as leadership, internal politics," reads one NSA document from dated April 18th, 2013.
The documents don't go into detail about how leading Russian politicians are monitored, such as whether their phones are tapped or information about their phone calls and internet use are registered. Nor is it clear if Russian President Vladimir Putin or other leaders are the target of the spying.
However, it appears the NSA is satisfied with the cooperation provided by FRA, which the US spy agency describes as "unique".
Ahead of a meeting with officials from FRA, NSA bosses are instructed to praise the Swedes, according to the investigative news programme.
“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 of the documents reviewed by SVT reads.
"FRA’s cable access has resulted in unique SIGINT reporting on all of these areas," it continues, using a common abbreviation to refer to signals intelligence.
According to UG, neither FRA or the NSA was willing to comment on the report.
"The quote you read here is the type of information that's hard for us to comment on," FRA spokesman Fredrik Wallin told SVT.
The NSA said only that "the US government has made clear that the United States gathers foreign intelligence of the type gathered by all nations".
The reports comes amid revelations about the extent of US-led international signals intelligence activities, with Sweden having been named previously as an important partner.
British journalist Duncan Campbell claimed earlier this year that Sweden, via FRA, had become "the biggest partner to (British intelligence agency) GCHQ outside the English-speaking countries".
Both FRA and the Swedish government have pointed out that Sweden's laws allow for international cooperation, but won't specify with which countries.
"A partner can't control us; what we cooperate on with lies within the framework of the direction the Swedish government has given us," Dag Hartelius, the recently installed head of FRA, said a month ago.
FRA is authorized to monitor cable-bound communications traffic to track "external threats" against the country. The intelligence gathering can only be directed toward foreign countries.
The government, military, Swedish Security Service (Säpo), and the National Bureau of Investigation (NBI) can order intelligence material from FRA. The agency can also share information with other countries.
Permits are authorized by a secret court, the Defence Intelligence Court (Försvarsunderrättelsedomstolen).