A European Union parliamentary delegation will meet United States security officials and members of Congress on Tuesday to convey concern and gather information after the latest revelations of America's spying on European leaders.
President Barack Obama has been in damage control, calling German Chancellor Angela Merkel - whose phone may have been bugged for more than a decade - and France's President Francois Hollande.
The leader of the delegation, British Labour MEP Claude Moraes, said an important priority was to discuss the impact of American spying on EU citizens' fundamental right to privacy.
The parliamentary visit will maintain public pressure for answers, but senior German intelligence officials who are being dispatched by Ms Merkel herself will demand much greater clarity behind the scenes, the BBC reports.
US officials have denied reports the chief of the National Security Agency (NSA) discussed with Mr Obama the alleged bugging of the German Chancellor's phone.
German media say the US has been tapping the Ms Merkel's phone since 2002, and that Mr Obama was told in 2010.
A report in German tabloid Bild am Sonntag claimed NSA chief General Keith Alexander had told the president about the bugging himself. An NSA source told the paper that Mr Obama had not stopped the operation, and had wanted to know all about Mrs Merkel as "he did not trust her".
However NSA spokeswoman Vanee Vines said General Alexander "did not discuss with President Obama in 2010 an alleged foreign intelligence operation involving Chancellor Merkel, nor has he ever discussed alleged operations involving Chancellor Merkel."
The statement does not make it clear whether the president was informed of the bugging operation by other means.
Mr Obama is reported to have told the German chancellor that he knew nothing of the operation when the two leaders spoke.
The spying allegations have caused outrage in Germany.
A report in Der Spiegel, based on leaked documents, said a US listening unit was based in its Berlin embassy and similar operations were replicated in 80 locations around the world. Such a listening post would be illegal under German law, according to Germany's interior minister.
A unit called Special Collection Services, based on the fourth floor of the US embassy in Pariser Platz in Berlin, was responsible for monitoring communications in the German capital's government quarter, including those targeting Mrs Merkel, the paper said.
Spanish media say 60 million calls monitored in month
Spanish media reports say the National Security Agency secretly monitored sixty 60 million calls there in just one month.
Newspapers say the details of the monitoring were included in documents provided by fugitive American intelligence analyst Edward Snowden. They say the NSA collected the numbers and locations of the caller and the recipient, but not the calls' content, the BBC reports.
The White House has so far declined to comment on Monday's claims about US spying in Spain, published in the newspapers El Pais and El Mundo.
It is alleged that the NSA tracked millions of phone calls, texts and emails from Spanish citizens between 10 December 2012 and 8 January this year.
The US ambassador to Madrid has been summoned to meet a Spanish foreign ministry official on Monday to discuss earlier allegations about US spying on Spanish citizens and politicians.
Claim Japanese help sought to spy on China
Government sources in Tokyo are quoted as revealing that the NSA asked the Japanese government two years ago help it monitor fibre-optic cables which carry personal data through Japan to the Asia-Pacific region.
The report says this was to allow the United States to spy on China - but Japan refused the request, the BBC reports.
The report on Kyodo's news agency cites an unnamed Japanese government source. The source said the NSA had cited the need to bolster information gathering in the face of China's growing presence in the cyberworld and the threat from international terrorism.
The source said the request was denied because Japan does not have the requisite legislation to intercept such communications - even if the aim is to prevent a terrorist act.