BERLIN - The German government on Monday invited the US ambassador in Berlin for talks, as European countries angry over reports that Washington bugged EU offices piled on to demand answers about the scope of US surveillance.

The Obama administration is facing a breakdown in confidence from key allies over secret surveillance programs that reportedly installed the covert listening devices in EU offices. Germany's move was its most direct expression of anger yet, and signalled that one of Washington's closest allies was unlikely to let the matter drop without at least a strong show of outrage.

"Eavesdropping on friends is unacceptable," German government spokesman Steffen Seibert told reporters in Berlin. "We're not in the Cold War anymore."

Some Europeans have warned that the bugging revelations could scuttle ongoing negotiations on a trans-Atlantic trade treaty, which seeks to create jobs and boost commerce by billions annually in what would be the world's largest free trade area.

Germany has been among the European countries most anxious to reach a trade deal with the US, and it will likely try to strike a careful balance in its criticism of Washington.

Europe's outage was triggered by a Sunday report by German news weekly Der Spiegel that the US National Security Agency bugged diplomats from friendly nations - such as the EU offices in Washington, New York and Brussels. The report was partly based on an ongoing series of revelations of US eavesdropping leaked by former NSA contractor Edward Snowden.

EU Foreign Affairs chief Catherine Ashton raised the issue with Secretary of State John Kerry on Monday during a meeting on the sidelines of a security conference in Brunei. Kerry had said he didn't know the details of the allegations, but maintained that many nations undertake lots of different kinds of activities to protect their national interests.

It's unclear how widespread similar practices actually are. But some in Europe have raised concerns that US efforts include economic espionage against its allies.

In Berlin, Seibert said German officials told their US counterparts over the weekend they were "alienated" by the reported bugging, and would seek details from the Obama administration.

NSA surveillance efforts were discussed by Chancellor Angela Merkel and US President Barack Obama when he visited Berlin two weeks ago. Merkel said at the time she made clear that "the issue of proportionality is always an important issue."

"Trust needs to be restored over this issue, especially in view of the trade negotiations just formally launched between the US and the 28-nation EU," Seibert said.

Italy also stepped up its criticism of the surveillance on Monday, with Foreign Minister Emma Bonino saying Italy had asked the Americans for the "necessary clarifications for this very thorny issue." In a statement, Bonino said the Americans had promised to provide clarification to both the EU and individual member states.

Italy had largely downplayed earlier reports of Snowden's revelations, even that the US had spied on G-20 members, in part because Italians are so used to being listened in on by their own government.

Italy is the most wiretapped Western democracy, with transcripts of telephone intercepts of politicians and criminals routinely splashed on front pages. Just this weekend, the phone intercepts of a top Vatican accountant arrested in a 20 million euro ($26.2 million) corruption plot were published in major Italian newspapers.

According to Der Spiegel, the NSA planted bugs in the EU's diplomatic offices in Washington and infiltrated the building's computer network. Similar measures were taken at the EU's mission to the United Nations in New York, the magazine said.

It also reported that the NSA used secure facilities at NATO headquarters in Brussels to dial into telephone maintenance systems that would have allowed it to intercept senior officials' calls and Internet traffic at a key EU office nearby.

The Spiegel report cited classified US documents taken by Snowden that the magazine said it had partly seen. It did not publish the alleged NSA documents it cited nor say how it obtained access to them.

The Guardian newspaper also published an article Sunday alleging NSA surveillance of the EU offices, citing classified documents provided by Snowden. The Guardian said one document lists 38 NSA "targets," including embassies and missions of US allies like France, Italy, Greece, Japan, Mexico, South Korea, India and Turkey.

In Athens, Greek Foreign Ministry spokesman Konstantinos Koutras said the Greek government could not understand why its missions should come under surveillance by "the services of a friendly and allied country" and was investigating the reports.

In Washington, a statement Sunday from the national intelligence director's office said US officials planned to respond to the concerns with their EU counterparts and through diplomatic channels with specific nations.

The furor is the latest in a nearly monthlong global debate over the reach of US surveillance that aims to prevent terror attacks. The two programs, both run by the NSA, pick up millions of telephone and Internet records that are routed through American networks each day. Reports about the programs have raised sharp concerns about whether they violate public privacy rights at home and abroad.