French President Emmanuel Macron said on Monday that wiretapping allies was not acceptable after reports that the US National Security Agency (NSA) had spied on European allies.
Mr Macron said France and Germany were seeking full clarity on what happened.
"This is not acceptable amongst allies", he told a news conference after a virtual Franco-German meeting with German Chancellor Angela Merkel.
It follows calls from other politicians for Denmark to explain why its foreign secret service allegedly helped the US spy on European leaders, including Ms Merkel, more than seven years ago.
The Danish broadcaster DR said the Danish Defence Intelligence Service, known in Denmark by its acronym FE, conducted in 2014 an internal investigation on whether the US NSA had used the co-operation with the Danes to spy against Denmark and neighbouring countries.
The probe concluded that the NSA had eavesdropped on political leaders and officials in Germany, France, Sweden and Norway.
According to DR, the alleged set-up between the United States and Denmark was codenamed Operation Dunhammer.
It allegedly allowed the NSA to obtain data by using the telephone numbers of politicians as search parameters.
DR said its report was based on interviews with nine unnamed sources, all of whom were said to have had access to classified information held by the FE. The military agency allegedly helped the NSA from 2012 to 2014.
She added the present government has “the same point of view” as the former Social Democratic government led by Helle Thorning-Schmidt, who was in power during that period: “the systematic wiretapping of close allies is unacceptable”.