The US National Security Agency (NSA) secretly monitored 60 million phone calls in Spain in one month, Spanish media say.
The reports say the latest allegations came from documents provided by the fugitive US analyst Edward Snowden.
They say the NSA collected the numbers and locations of the callers and the recipients, but not the calls' content.
This comes as an EU parliamentary delegation is due to meet officials in Washington to convey concerns.
The officials from the European Parliament's Committee on Civil Liberties, Justice and Home Affairs are shortly expected to speak to members of the US Congress to gather information.
In Madrid, the US ambassador to Spain vowed to clear the "doubts" that had arisen about his country's alleged espionage after a meeting with the Spanish EU minister, the Efe news agency reports.
During the meeting, the minister said such practices, if true, were "inappropriate and unacceptable from partners and friendly countries".
It is not clear how the alleged surveillance was carried out, whether it was from monitoring fibre-optic cables, data (including metadata) obtained from telecommunication companies, or other means.
In emails, the data collected is usually the sender and recipient email addresses, their IP addresses, the message file size, and sometimes the top or subject line of the message.
In telephone calls, it means the numbers of the caller and recipient, its duration, time, date and location (for mobiles, determined by which mobile signal towers relayed the call or text).
The contents of the telephone conversation itself, however, are not covered, US intelligence officials say. The NSA has suggested it does not usually store the geolocational information for mobile phone calls.
Meanwhile, a Japanese news agency says the NSA asked the Japanese government in 2011 to help it monitor fibre-optic cables carrying personal data through Japan, to the Asia-Pacific region.
The reports, carried by the Kyodo news agency, say that this was intended to allow the US to spy on China - but Japan refused, citing legal restrictions and a shortage of personnel.
It is alleged that the NSA tracked millions of phone calls, texts and emails from Spanish citizens between 10 December 2012 and 8 January 2013.
The allegations follow German media reports that the US was bugging Chancellor Angela Merkel's phone for more than a decade - and that the surveillance only ended a few months ago.
Mrs Merkel is sending her country's top intelligence chiefs to Washington this week to "push forward" an investigation into the spying allegations, which have caused outrage in Germany.
Britain's Guardian newspaper reported on Friday that the NSA had monitored the phones of 35 world leaders. Again Mr Snowden was the source of the report.
The head of the European Parliament's delegation, British MEP Claude Moraes, told the BBC it was the scale of the NSA's alleged surveillance that was worrying.
"The headline news, that 35 leaders had their phones tapped, is not the real crux of the issue," he said.
"It really is the El Mundo type story, that millions of citizens of countries... had their landlines and other communications tapped. So it's about mass surveillance. It's about scale and proportionality."
He said a priority of the European mission was to discuss the impact of American spying on EU citizens' fundamental right to privacy.
The BBC's Europe correspondent Chris Morris says that with every new allegation, demands are growing in Europe - and in Germany in particular - for explanations and for guarantees of a change in culture.
EU leaders have said that distrust of the US over spying could harm the fight against terrorism.