The United States on Thursday refused to break its silence about a spying row which led a furious Germany to expel the CIA station chief in Berlin, but concern deepened among lawmakers about damage to relations with Europe's dominant power.
Germany's decision was a stunning show of discord between two such close allies and a signal that Washington has failed to quell anger over revelations about its espionage tactics in Germany, which has been building for more than a year.
But in contrast with the uproar in Berlin, Washington responded coolly, demurring when asked to comment on Germany's shock move, choosing to instead stress the value of its relationship with Berlin.
White House spokesman Josh Earnest said in Texas, where President Barack Obama was on the road, that any "sort of comment on any reported intelligence acts would put at risk US assets, US personnel and United States national security."
He said that the intelligence relationship between the two countries was crucial to the security of both Americans and Germans and that contacts with Berlin on the issue were taking place through diplomatic, intelligence and law enforcement channels.
National Security Council spokeswoman Caitlin Hayden said it was "essential that cooperation continue in all areas" with Germany.
State Department spokeswoman Jen Psaki said that US secretary of state John Kerry and German foreign minister Frank-Walter Steinmeier would likely speak in the coming days.
Rift between key allies
US officials habitually decline to discuss intelligence issues in public.
But Germany's move elevated the spying row to a political and diplomatic confrontation between the United States, leader of the West, and Europe's most influential power -- as both nations work together to confront global crises, including the showdown with Russia over Ukraine.
Transatlantic intelligence cooperation between the United States, Germany and other key European nations is also crucial to detecting and subverting terror plots, as fears rise that Muslim radicals from Syria holding Western passports could be plotting new attacks.
The unflustered US stance was likely to do little to quell outrage in Germany, where there are demands for a public sign of US contrition.
Anger over US espionage was first stirred last year by revelations of mass National Security Agency (NSA) data collection on German communications networks by fugitive leaker Edward Snowden.
Claims that Washington had tapped German chancellor Angela Merkel's mobile phone took the row to a new level and ill-feeling had barely subsided when German authorities arrested a man reportedly accused of being a double agent for the CIA.
With Merkel under immense political pressure, German patience snapped amid claims a second government official was being investigated for allegedly passing secrets to the United States.
US lawmakers worried
The sudden escalation of the crisis on Thursday sparked alarm among top lawmakers at the wider diplomatic damage the spy row was beginning to wreak.
Democrat Dianne Feinstein, who chairs the Senate Select Committee on Intelligence, said she was "deeply concerned" by the episode, but could not talk about it.
Another committee member, Republican Senator Jim Risch, told AFP that "this is a situation that's starting to get out of hand."
"The executives of both countries need to sit down at a table and try to get this resolved, so that this thing doesn't get further away from where we are," Risch said.
"Germany is an important country to us, has been for a long time from an economic standpoint, from a trade standpoint, and look, they're the foundation right now for Europe, that really kind of binds the European Union together."
One of President Barack Obama's earliest political allies, Democratic Senator Tim Kaine of Virginia, told AFP that the row with Germany was "very discouraging."
"I think the US administration has to sit down in face-to-face meetings with Germany and try to find a path going forward.
"I definitely think the president should deal directly with Angela Merkel on this. I think it's very important we do, because the relationship is too important to let it fray over this."
The White House has so far responded to questions about whether Obama will talk directly with Merkel on the issue by saying no such contacts are scheduled.
Officials have said that Obama did not know about the new spying affair when he spoke by telephone to Merkel a week ago.
The two leaders have had a close relationship, and Obama has made know Merkel is one of the world leaders he most respects.
But the political pressure on the German leader is intense over the issue -- especially after Washington declined to agree to conclude a "no spy" deal with Germany, similar to the ones it has with allies Britain, Australia, Canada and New Zealand.