Germany has summoned the U.S. ambassador in Berlin to answer questions over whether America may have monitored German Chancellor Angela Merkel's phone calls.
The issue of U.S. surveillance in Europe looks set to dominate a summit of the European Union's (EU) 28 leaders in Brussels later Thursday.
The meeting between German Foreign Minister Guido Westerwelle and U.S. John B Emerson later Thursday comes a day after Merkel called President Barack Obama to demand clarification about whether her phone had been tapped. Merkel warned that such an action would amount to a "serious breach of trust", according to government spokesperson Steffen Seibert.
But the White House insisted in a statement that it "is not monitoring and will not monitor" Merkel's phone.
French President Francois Hollande has been pushing for the spying revelations to be put on the agenda when EU heads of state meet in Brussels Thursday for a two-day summit. Documents released by former NSA contractor Edward Snowden revealed that 70 million phone calls made in France over the course of 30 days were recorded, according to a report in Le Monde.
An EU Council spokesperson told CNBC that the topic was not a specific agenda item, but the government leaders will be able to take the conversation in that direction.
Meanwhile, members of the European Parliament have been in uproar in the past few months over reports in German newspaper Der Spiegel that the U.S.'s National Security Agency (NSA) bugged EU offices.
The lawmakers have staged a series of inquiries into the spying allegations. On Wednesday, they voted in favour of suspending a controversial financial data sharing agreement between the European Union (EU) and the U.S. aimed at tracking terrorists' funds called the Terrorist Finance Tracking Program (TFTP).
(Read more: US spying a 'slap in the face': EU lawmaker)
This was in response to other reports that those U.S. authorities are monitoring personal money transfer information, including bank and credit card transactions, from a Brussels-based system called SWIFT, which collates global financial transactions.
But EU home affairs Commissioner Cecilia Malmström said the EU-U.S. data-sharing agreement would remain and that there are "no indications that the TFTP Agreement has been violated".
The move by European politicians to question the U.S. surveillance program is an important moment in the EU-US relationship according to a think tank.
"I think it follows an approach that the European Union has been observing in the last few months to look into the relationship between the EU and U.S. in a very thorough way," Paolo Balboni, scientific director at the European Privacy Association, told CNBC.
"It is more evidence that the EU is putting US surveillance under the microscope, which I think is a good approach because we need to know what is going on."
The U.S. has created hostility around the world with Snowden's spying allegations, first unveiled in The Guardian newspaper.
Italian Prime Minister Enrico Letta discussed claims of U.S. snooping with US Secretary of State John Kerry on Wednesday during talks in Rome.
Meanwhile Brazil's President Dilma Rousseff cancelled a visit to Washington following allegations that the U.S intercepted her emails and messages.