Norway said on Thursday it had summoned the US embassy's top official in Oslo to lodge an official protest following a report that Washington had spied on Norwegian and other European leaders.

"The defence ministry held a meeting with the US embassy in Oslo today where we made it clear that spying on allies is unacceptable and unnecessary," Norway's Defence Minister Frank Bakke-Jensen said on Twitter.

The ministry said the US charge d'affaires -- Richard Riley, according to the embassy's website -- was the person who attended the meeting with a senior Norwegian official.

The US embassy is currently without an ambassador.

In an investigative report on Sunday, Danish public broadcaster Danmarks Radio (DR) said the US National Security Agency (NSA) had eavesdropped on Danish underwater internet cables from 2012 to 2014.

They spied on top politicians in France, Germany, Norway and Sweden, including German Chancellor Angela Merkel.

Paris, Berlin and other European capitals have demanded answers from Washington and Copenhagen, though reports of allies spying on each other have surfaced ever since the Snowden affair in 2013.

The NSA got access to text messages, telephone calls and internet traffic including searches, chats and messaging services -- including those of Germany's Chancellor Angela Merkel, then-foreign minister Frank-Walter Steinmeier and then-opposition leader Peer Steinbruck, DR said.

"I'm pleased that the Americans clearly said that they changed their practices in 2014 when it comes to the monitoring of allies, and that they want to cooperate with us and others to chart what happened," Norwegian news agency NTB quoted Norwegian Prime Minister Erna Solberg as saying.

"We summoned the US embassy in Oslo today to follow up on this," she added.

According to NTB, Solberg also held talks on Thursday with her Danish counterpart Mette Frederiksen.

"I reiterated that we consider espionage against close friends and allies unacceptable and unnecessary," she said.