A top US diplomat has tried to play down the damage to Washington's diplomacy in Ukraine from a leaked telephone call.
However, German Chancellor Angela Merkel called an obscene remark about the European Union "absolutely unacceptable."
US officials blamed Moscow for the internet leak of recordings of a senior State Department official and the US ambassador discussing a possible future government for Ukraine
Western officials described the leaks as a throwback to the cloak-and-dagger tactics of the Cold War, apparently aimed as much at sowing discord among Western allies as at discrediting the opposition in Ukraine.
Senior State Department official Victoria Nuland is heard using an expletive to tell the US ambassador it would be better if a new Ukrainian government is backed by the United Nations than the EU.
"F**k the EU," she says.
US officials have not denied the authenticity of the recording and said Ms Nuland apologised to EU colleagues for the comment.
Ms Merkel - already furious with Washington for several months over reports that US officials bugged her own phone - found Ms Nuland's remarks "totally unacceptable", a spokeswoman for the German chancellor said.
Ms Merkel also expressed support for EU foreign policy chief Catherine Ashton, who heads the bloc's Ukraine policy.
In a separate leaked recording, an Ashton aide is overheard complaining about the United States for telling Ukrainian opposition members that Brussels was "soft" in its reluctance to impose measures such as sanctions to hurt the pro-Russian government.
Ms Nuland, who met President Viktor Yanukovych in Kiev yesterday before the Ukrainian leader left to meet President Vladimir Putin at the Winter Olympics in Russia, described the bugging and leaks as "pretty impressive tradecraft" but said it would not hurt her ties with the Ukrainian opposition.
Other US officials directly blamed Moscow, noting that the leaked recording, posted anonymously, was first highlighted in a tweet from a Russian official.
"Tapping phone calls and releasing carefully selected bits to support propaganda efforts is an age-old method by some type of regimes," tweeted Swedish Foreign Minister Carl Bildt.
In Ms Nuland's bugged call, apparently recorded about 12 days ago when Ukrainian opposition leaders were considering an offer from Ms Yanukovych to join his cabinet, she suggested that one of three leading figures might accept a post but two others should stay out.
In the end, all three rejected the offer.