Media in Europe say a US intelligence service in partnership with Denmark spied on senior European politicians, including German Chancellor Angela Merkel, between 2012 and 2014.
Media outlets, including Denmark's public broadcaster DR, reported that the US National Security Agency cooperated with a Danish intelligence service for the spying.
They say the NSA eavesdropped on political leaders and officials in Germany, France, Sweden, and Norway.
The NSA is suspected of gathering the data by using the communication networks of Denmark that hosts landing stations for subsea internet cables to and from neighboring countries.
Disclosure of confidential documents in 2013 by former NSA contractor Edward Snowden raised suspicion that the United States wiretapped Merkel and other officials.
The revelation prompted Merkel to ask the US for clarification, saying wiretapping between allies was unacceptable.
At a news conference on Monday, Merkel asked for a clarification of the facts, saying that her country's position has not changed.
French President Emmanuel Macron agreed with Merkel and asked the US and Denmark for explanations.