Washington's new top diplomat for Europe, Victoria Nuland, has apologised to EU counterparts after she was caught cursing the European response to the crisis in Kiev.
US officials, while not denying such a conversation took place, didn't go into details about the apology, and pointed the finger at Russia for allegedly bugging the diplomats' phones.
A spokeswoman for German Chancellor Angela Merkel said she sees Nuland's comments as "absolutely unacceptable".
"The chancellor considers this statement absolutely unacceptable ... and wants to emphasise again that (EU foreign policy chief Catherine) Ashton is doing an outstanding job," she said.
I would say that since the video was first noted and tweeted out by the Russian government, I think it says something about Russia's role.
A video, titled the "Marionettes of Maidan" referencing the centre of protests in Kiev, was published on YouTube on February 4th, and shows photos of Nuland and the US ambassador to Ukraine, Geoffrey Pyatt.
In the audio, voices resembling those of Nuland and Pyatt discuss
international efforts to resolve Ukraine's ongoing political crisis.
At one point, the Nuland voice colourfully suggests that the EU's position should be ignored.
"F--- the EU," the female voice said.
A US State Department official, speaking on condition of anonymity because he was not authorised to publicly discuss the matter, told the Associated Press news agency that it sounds like an authentic recording of a call that occurred last week.
Nuland refused to comment on the video during a press conference following talks with the opposition and President Vikto Yanukovych.
"I will not comment on a private diplomatic conversation," she said.
"It was pretty impressive tradecraft. (The) audio quality was very good," she added.
State Department spokeswoman Jen Psaki said on Thursday that Nuland "has been in contact with her EU counterparts and of course has apologised for these reported comments".' But Psaki did not reveal to whom Nuland apologised on Thursday.
US blames Russia
The leaked phone call appears to reveal US frustration with the EU over handling Ukraine, which is torn between leaning to the European Union and its past master Russia.
US officials pointed out that an aide to Russian deputy prime minister, Dmitry Rogozin, was among the first to tweet about the video that contains audio subtitled in Russian of the alleged call.
In the tweet, posted some seven hours before existence of the video became widely known on Thursday, Loskutov, said: "Sort of controversial judgment from Assistant Secretary of State Victoria Nuland speaking about the EU."
White House spokesman Jay Carney pointed to the tweet and Russia's clear interest in what has become a struggle between pro-Moscow and pro-Western camps in the former Soviet Republic, but declined to comment on the source of the audio.
"I would say that since the video was first noted and tweeted out by the Russian government, I think it says something about Russia's role," Carney told a news conference.
He would not comment on the substance of the conversation, in which the Nuland and Pyatt voices also discuss their opinion of various Ukrainian opposition figures.
State Department spokeswoman Jen Psaki said that if the Russians were responsible for listening to, recording and posting a private diplomatic telephone conversation, it would be "a new low in Russian tradecraft".