BERLIN - Germany's Angela Merkel has said a US official's apparent insult of the EU's efforts to mediate in the Ukraine crisis is "totally unacceptable".
Victoria Nuland has apologised after a voice resembling hers used a graphic swear word in a conversation said to be with the US ambassador to Ukraine.
A recording of the apparently bugged conversation was posted online, with the US hinting at Russia's involvement.
The EU and US are involved in talks to end months of unrest in Ukraine.
After meeting Ukraine's President Viktor Yanukovych in Kiev, Ms Nuland - US Assistant Secretary of State - said she would not make a public statement on the matter.
"I will not comment on a private diplomatic conversation," she told reporters.
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 using its economic clout to persuade Mr Yanukovych not to pursue closer ties with Brussels.
Russia has itself accused Washington and the EU of meddling in Ukraine.
'Not a good idea'
German spokeswoman Christiane Wirtz said Ms Merkel fully supported the work EU policy chief Catherine Ashton had been doing to find a solution to the crisis.
"The chancellor finds these remarks totally unacceptable and wants to emphasise that Mrs Ashton is doing an outstanding job," Ms Wirtz said.
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 attempts 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.