The German chancellor, Angela Merkel, has described as "totally unacceptable" remarks by a senior US official who said "fuck the EU" while speaking about the crisis in Ukraine.

In a leaked conversation posted on YouTube, the state department official Victoria Nuland revealed the White House's frustrations at Europe's hesitant policy towards pro-democracy protests in Ukraine, which erupted late last year. Nuland was talking to the US ambassador to Ukraine, Geoffrey Pyatt.

The German spokeswoman Christiane Wirtz said Merkel appreciated the work of Catherine Ashton, the EU's foreign policy chief, who had tried to mediate between the Ukrainian president, Viktor Yanukovych, and protesters who have taken to the streets. "The chancellor finds these remarks totally unacceptable and wants to emphasise that Mrs Ashton is doing an outstanding job," Wirtz said.

Speaking in Kiev, after meeting Yanukovych, Nuland refused to be drawn into the row. "I will not comment on a private diplomatic conversation," she said. But she implied she had been a victim of a sophisticated eavesdropping operation carried out by Russia's spy agencies. The embarrassing tape surfaced with Russian subtitles. "It was pretty impressive tradecraft. [The] audio quality was very good," Nuland said on Friday.

Earlier, US officials also pointed the finger at the Kremlin. They noted a tweet from Dmitry Loskutov, an aide to the deputy prime minister of Russia, Dmitry Rogozin, that said: "Sort of controversial judgment from assistant secretary of state Victoria Nuland speaking about the EU."

Loskutov, however, on Friday denied that either he or the Russian government had leaked the tape. He said he had stumbled across it while surfing a social networking website, the Associated Press reported. Its Russian title reads "Marionettes of the Maidan", a reference to the square in Kiev where anti-government demonstrators and riot police have faced off for weeks. The recording appears to have been released to drive a wedge between the EU and Washington, and to discredit Ukraine's opposition as US stooges.

The White House spokesman Jay Carney would not discuss the content of the conversation recorded in the clip, but he too invoked the Loskutov tweet. "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," he said.

Blaming the Russians for leaking a conversation that was presumably obtained by covert means poses problems for the US, as documents leaked by Edward Snowden reveal that the US has in the past listened into the communications of its allies, as well as enemies.

The conversation underlines mounting US frustration at the EU's position on the ongoing democracy protests in Ukraine. The EU has held back from joining US threats to impose sanctions should the Ukrainian regime violently suppress the protests.

In the tapes, Nuland and Pyatt discuss the upheavals in Ukraine, and Yanukovych's offer last month to make the opposition leader Arseniy Yatsenyuk the new prime minister and Vitali Klitschko deputy prime minister. Both men turned the offer down.

Nuland, who in December went to Independence Square in Kiev in a sign of support for the demonstrators, adds that she has also been told that the UN chief, Ban Ki-moon, is about to appoint a former Dutch ambassador to Kiev, Robert Serry, as his representative to Ukraine.

"That would be great I think to help glue this thing and have the UN glue it and you know, fuck the EU," she says, in an apparent reference to differences over their policies.

"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," Pyatt replies.

In the phone call, Nuland suggests that Klitschko, the former world champion boxer, is not yet suited to take a major government role, in contrast to Yatsenyuk.

"I don't think Klitsch should go into the government," she apparently said.

"I think Yats is the guy who's got the economic experience, he's got the governing experience," she adds.

A spokeswoman for Klitschko said: "Our position is, it does not make sense to comment on conversations whose authenticity has not been confirmed by anyone

• AFP and the Associated Press contributed to this report