The four documents, which were leaked by NSA whistleblower Edward Snowden, were published on Thursday by Denmark's Information newspaper in partnership with The Intercept, the internet news site set up by Glenn Greenwald, the US journalist who received Snowden's leaks.
In one of the documents, a set of briefing notes for a meeting between the then NSA head Keith Alexander and his Danish counterparts, Alexander is told to "remind the Danes of the long NSA-DDIS partnership working cable access with".
This appears to indicate that the Danes are accessing cables in partnership with the NSA. The document also refers to the NSA helping DDIS "manage" its "special access".
The Intercept sees the new documents as demonstrating that the NSA works closely with Denmark and other countries, as well as with the so-called Five Eyes surveillance alliance of the United Kingdom, Canada, New Zealand, and Australia.
"A number of other countries, described by the NSA as “third-party partners,” are playing an increasingly important role – by secretly allowing the NSA to install surveillance equipment on their fiber-optic cables," the site wrote in its article, which was published simultaneously with Information's.
Danish Defence Minister Nicolai Wammen refused to comment on the specific allegations, but stressed that any actions taken by DDIS were within Danish law.
"We are a country that is targeted by terrorists and also from others who do not have a positive agenda when it comes to the kingdom. Therefore, it is in Denmark's interest that our intelligence services cooperate with other countries' intelligence services," he said.
According to the Intercept, governments that participate in RAMPART-A are rewarded with "access to the NSA’s sophisticated surveillance equipment, so they too can spy on the mass of data that flows in and out of their territory."
The also receive a commitment from the NSA that the intercepted data will not be used to spy on their citizens, with some exceptions.