The head of the European Council says U.S. Assistant Secretary of State Victoria Nuland's use of vulgar language about the European Union is "totally unacceptable."
Council president Herman Van Rompuy commented during a Saturday interview with Belgian broadcasters, a day after German Chancellor Angela Merkel echoed a similar sentiment.
Nuland's comment was part of a private phone conversation that was leaked to social media.
In the call with U.S. Ambassador to Ukraine Geoffrey Pyatt, Nuland used foul language to suggest that the United States should ignore the European Union's position on resolving the crisis in Ukraine.
Nuland said Friday she would not talk about what she called a "private diplomatic conversation."
But in a Friday briefing, U.S. State Department spokeswoman Jen Psaki said Nuland had apologized for the comment.
"She would convey to you that she apologized obviously because that's not, doesn't reflect how she feels about our relationship with the EU. It is also important to note that she has been in close touch with EU officials since then. Not about this, but about work we're doing together on Ukraine."
U.S. officials have implied Russia may be involved in the leak, which resulted in Nuland's remarks being posted on the YouTube website.
U.S. officials on Thursday pointed to a tweet by an aide to Russian Deputy Prime Minister Dmitry Rogozin as the first to promote the recording.
In comments to the Associated Press, Rogozin denied involvement, saying he merely repeated a posting he found on a social media website.
Nuland said the recording of her conversation was "impressive tradecraft" - a term referring to activity by intelligence agents.
Ukrainian President Viktor Yanukovych and Russian President Vladimir Putin met behind closed doors in the Olympic city of Sochi on Saturday, but a Ukrainian official would not give any details about the meeting.
Yanukovych triggered mass protests in November when he yielded to Russian pressure and backed out of a free trade pact with the EU. Protesters continue to camp out in Kyiv.