French President Francois Hollande Monday told the United States immediately to stop spying on its European allies after Secretary of State John Kerry promised to probe the allegations but said information-gathering was "not unusual" in international affairs.
"We cannot accept this kind of behaviour between partners and allies," Hollande told journalists during a visit to the northwestern French city of Lorient. "We ask that this immediately stop."
Kerry was forced to confront the controversy that was triggered by fugitive leaker Edward Snowden during direct talks with European Union foreign policy chief Catherine Ashton at an Asia-Pacific security forum in Brunei.
"Lady Ashton did indeed raise it with me today and we agreed to stay in touch. I agreed to find out exactly what the situation is and I will get back to her," Kerry told reporters.
Kerry said that, due to an intense few days of Middle East peace negotiations before coming to Brunei on Monday, he was unaware of the details of the allegations that the US National Security Agency (NSA) bugged European offices and embassies.
"(But) I will say that every country in the world that is engaged in international affairs, of national security, undertakes lots of activities to protect its national security and all kinds of information contributes to that," he said.
"All I know is that is not unusual for lots of nations. But beyond that, I'm not going to comment any further until I have all of the facts and find out precisely what the situation is."
Kerry's comments were the most senior official reaction from the US government to the allegations, which threaten to seriously harm relations between the United States and its European allies.
European Justice Commissioner Viviane Reding said a long-awaited deal between the European Union and the United States could be in jeopardy, while Germany's justice minister likened the alleged espionage to spying on Cold War enemies.
In fresh revelations attributed to Snowden, a former NSA contractor now holed up at Moscow airport, Monday's Guardian newspaper said France, Italy and Greece were among 38 "targets" of spying operations by US intelligence services.
This came after a report in German weekly Der Spiegel detailed alleged covert surveillance by the NSA on EU diplomatic missions.
One document, dated September 2010 and classed as "strictly confidential", described how the NSA kept tabs on the EU's mission in Washington, Der Spiegel reported.
Microphones were allegedly installed in the building and the computer network was infiltrated, giving the agency access to emails and internal documents.
Der Spiegel said the EU delegation at the United Nations was subject to similar surveillance and the spying had also extended to the 27-member bloc's Brussels headquarters.
Reding warned that talks to create what would be the world's biggest free trade area could be jeopardised if the bugging allegations proved true.
"We can't negotiate a large trans-Atlantic 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.
The talks were only launched last month.
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.
"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."
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 the only previous US reaction to Der Spiegel's claims, Deputy National Security Advisor Ben Rhodes, while refusing to comment directly on the allegations, said Saturday it was "worth noting" the US was "very close" to EU security services.
The US authorities issued an arrest warrant for Snowden last month after he revealed details of the NSA's PRISM programme.
It 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 is stranded and in political limbo at the transit area of Moscow's Sheremetyevo airport after arriving from Hong Kong last week, unable to fly on without legal travel documents or exit the airport without a Russian visa.