German chancellor Angela Merkel says a United States official's insult of the European Union's efforts to mediate in the Ukraine crisis is "totally unacceptable".

The US Assistant Secretary of State for European Affairs, Victoria Nuland, has apologised after referring disparagingly to the EU's role during a conversation said to be with the US Ambassador to Ukraine, Geoff Pyatt.

US officials have not denied the authenticity of the recording, which comes as the EU and US hold talks to end months of unrest in Ukraine.

Mass anti-government protests erupted in the country in November after president Viktor Yanukovych refused to sign a trade agreement with the EU.

In the recording Ms Nuland and US ambassador to Kiev, Geoff Pyatt, discuss frankly which opposition figures should go into the new Ukrainian government.

"That would be great I think to help glue this thing and have the UN glue it and you know, f--- the EU," Nuland says, in apparent frustration at policy differences.

US officials have blamed Moscow for the leak of Ms Nuland's phone call.

A recording of the apparently bugged exchange was posted anonymously on YouTube, but was first highlighted in a tweet from a Russian official.

Dr Merkel, already furious with Washington for several months over reports that US officials bugged her own phone, voiced her anger and expressed support for EU foreign policy chief Catherine Ashton, who heads the bloc's Ukraine policy.

"The chancellor considers this statement absolutely unacceptable ... and wants to emphasise again that (EU foreign policy chief Catherine) Ashton is doing an outstanding job," her spokeswoman said.

"The European Union will continue with its intensive efforts to calm the situation in Ukraine."

In a separate leaked recording, an Ashton aide is overheard complaining about the US for telling Ukrainian opposition members that Brussels was "soft" in its reluctance to impose measures such as sanctions.

Ms Nuland has described the bugging and leaks as "pretty impressive tradecraft" but said it would not hurt her ties with the Ukrainian opposition.

Swedish foreign minister Carl Bildt, meanwhile, tweeted: "Tapping phone calls and releasing carefully selected bits to support propaganda efforts is an age-old method by some type of regimes."

ABC/wires