An apparently bugged phone conversation in which a senior US diplomat disparages the EU over the Ukraine crisis has been posted online.
A voice resembling that of Assistant Secretary of State Victoria Nuland refers to the EU using a graphic swear word, in a conversation apparently with the US ambassador to Ukraine.
The US said Ms Nuland had "apologised for these reported comments".
The EU and US are involved in talks to end months of unrest in Ukraine.
Mass anti-government protests erupted in Ukraine in late November after President Viktor Yanukovych refused to sign a far-reaching association and trade agreement with the EU - under heavy pressure from Moscow.
Russia has been widely accused of intervening in Ukraine, using its economic clout to persuade Mr Yanukovych to abandon closer ties with Brussels.
Russia has itself accused Washington and the EU of meddling in Ukraine.
'Not a good idea'
The alleged conversation between Ms Nuland and the US ambassador to Ukraine, Geoffrey Pyatt, appeared on YouTube on Thursday.
The 4min 10sec video was entitled "Maidan's puppets" in Russian - a reference to the square in the Ukrainian capital, Kiev, where pro-EU protests have been held for months. A transcription of the whole conversation was also posted in Russian.
At one point, the female speaker mentions the UN and its possible role in trying to find a solution to the Ukraine stand-off.
She says: "So that would be great, I think, to help glue this thing and have the UN help glue it and you know..." she then uses the graphic swear word about the EU.
The male replies: "We've got to do something to make it stick together, because you can be pretty sure that if it does start to gain altitude the Russians will be working behind the scenes to try to torpedo it."
The two officials also discuss frankly the merits of the three main Ukrainian opposition leaders - Vitaly Klitschko, Arseniy Yatseniuk and Oleh Tyahnybok.
The female speaker says that Mr Klitschko, the former heavyweight boxing world champion, should not be in any new government. "I don't think it's a good idea."
She adds: "I think Yats (Arseniy Yatseniuk) is the guy who's got the economic experience."
US officials refused to confirm or deny the tape's authenticity, but state department spokeswoman Jan Psaki said: "I didn't say it was inauthentic."
Ms Psaki said Ms Nuland had "been in contact with her EU counterparts and of course has apologised for these reported comments".
An EU official told the BBC: "The EU is engaged in helping the people of Ukraine through the current political crisis. We don't comment on alleged leaked telephone conversations."
Ms Psaki also played down the comments about Ukraine's opposition, saying: "It shouldn't be a surprise that at any points there have been discussions about recent events and offers and what is happening on the ground."
Ms Psaki hinted that the tape could have been leaked by Moscow, pointing out that a senior Russian official was one of the first to draw attention to the audio.
She said: "We think this is a new low in Russian trade-craft. This is something they've been actively promoting, posting on, tweeting about."
White House spokesman Jay Carney added: "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."
Earlier on Thursday, a senior aide to Russian President Vladimir Putin accused Washington of interfering in Ukraine's domestic affairs.
Sergei Glazyev said the US was spending $20m (£12.3m; 14.8m euros) a week on Ukrainian opposition groups, supplying "rebels" with arms among other things.
And he suggested that Moscow could also intervene.
Mr Yanukovych held talks in Kiev with Ms Nuland on Thursday, at which he said he favoured dialogue and compromise with the opposition.
The Ukrainian leader is to meet President Putin on Friday on the sidelines of the Winter Olympics in Sochi.
Moscow has promised a $15bn (£9.2bn) loan to Ukraine but said it would not be released in full until the formation of a new government in Kiev.
Ukrainian Prime Minister Mykola Azarov and his cabinet resigned last month as President Yanukovych sought to calm street protests.
Also in Kiev, thousands of Ukrainian opposition activists, some carrying shields and baseball bats, marched from their camp on Independence (Maidan) Square to parliament in a show of force.
They came close to government supporters who are camped next to parliament behind barricades manned by hundreds of police, but the march passed off peacefully.