A long-awaited trade deal between the European Union and the United States could also be in jeopardy over allegations that Washington bugged EU offices, European Justice Commissioner Viviane Reding warned on Sunday.
Brussels, Paris and Berlin reacted angrily to a report in German weekly Der Spiegel on Sunday which detailed covert surveillance by the US National Security Agency (NSA) on EU diplomatic missions.
The report was based on confidential documents, some of which it had been able to consult via Snowden.
Reding warned that talks to create what would be the world's biggest free trade area, formally launched earlier this month, could be jeopardized if the bugging allegations proved true.
"We can't negotiate a large transatlantic market if there is any doubt that our partners are bugging the offices of European negotiators," Reding said at a meeting in Luxembourg, her spokesperson told AFP.
"We have immediately been in contact with the US authorities in Washington DC and in Brussels and have confronted them with the press reports," the European Commission said in a statement.
The US said on Sunday it would respond to the EU via diplomatic channels over the bugging allegations. A representative of the US embassy in Rome told The Local on Monday that they had not received an official response from Washington.
"While we are not going to comment publicly on specific alleged intelligence activities, as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations," said a statement from the office of the Director of National Intelligence in Washington.
One document, dated September 2010 and classed as "strictly confidential", describes how the NSA kept tabs on the European Union's mission in Washington, Der Spiegel said.
According to documents seen by the Guardian, bugs were implanted on the encrypted fax machine at the embassy as part of operation 'Perdido', set up to learn about rifts between member nations.
The EU delegation at the United Nations was subject to similar surveillance, Der Spiegel said, adding that the spying also extended to the 27-member bloc's Brussels headquarters.
The files also revealed that, in addition to the EU, the US embassies of France, Greece and Italy were among 38 "targets" of NSA spying operations, Monday's Guardian reported.
In the only US reaction to the Spiegel claims so far, Deputy National Security Advisor Ben Rhodes, while refusing to be drawn into commenting directly on the allegations, said on Saturday it was "worth noting" the US was "very close" to EU security services.
The reports are the latest in a series of allegations about US spying activity revealed by Snowden, a former NSA contractor.
He is now stranded at a Moscow airport transit zone looking for a country to accept his asylum request after the United States issued a warrant for his arrest and revoked his passport.
EU powerhouse Germany said the United States must quickly say whether the spying allegations were true or not.
"It's beyond our imagination that our friends in the US consider the Europeans as enemies," Justice Minister Sabine Leutheusser-Schnarrenberger said in a statement.
"If the media reports are accurate, it is reminiscent of actions among enemies during the Cold War."
French Foreign Minister Laurent Fabius said Paris had also demanded an explanation from US authorities. Such spying activities, if confirmed, would be "totally unacceptable", he said.
European Parliament president Martin Schulz said in a statement he was "deeply worried and shocked" by the reports.
"If the allegations prove to be true, it would be an extremely serious matter which will have a severe impact on EU-US relations."
In its latest report on Sunday, Der Spiegel said leaked documents showed that the US secret services had targeted Germany more than any other EU country.
Citing figures from NSA documents, the magazine said that half a billion forms of communication - phone calls, emails, text messages and Internet chat entries - were monitored in Germany every month.
The US authorities issued an arrest warrant this month for Snowden after he revealed details of the NSA's so-called PRISM programme which collects and analyses information from Internet and phone users around the world, with access to data from Google, Yahoo! and other Internet firms.
US officials say the information gathered is vital in the fight against global terrorism, but the scale of the programme raised deep concerns around the world.
Snowden himself remains in political limbo at Moscow's Sheremetyevo airport after flying in from Hong Kong last week, unable to fly on without legal travel documents or exit the airport without a Russian visa.
Ecuador's President Rafael Correa said US Vice President Joe Biden had asked Quito to reject any asylum request from the 30-year-old. Washington wants to put him on trial on charges including espionage.
Correa said Snowden's fate was in Russia's hands as Quito could not process his asylum request until he was on Ecuadoran soil.