William Hague, the shadow foreign secretary, defended the Conservatives' alliance with far-right parties in Europe.
At a meeting in Washington with US secretary of state, Hillary Clinton, Hague said that the accusations about the Conservatives' alliances were part of a smear campaign by the New Statesman, the Guardian, and others.
Hague's visit has been clouded by a row in Washington over these partners - including a Latvian party with members who have taken part in commemoration servies for the Waffen-SS, and a Polish party with members who have questioned the need to apologies for an anti-Jewish pogrom.
Jewish groups in the US and in Europe have expressed anger about the Tories' alliance.
Speaking to the media, Hague said: "I have assured her [Clinton] they are mainstream parties of the centre-right ... The Conservative party rejects what it regards as a smear campaign against some of its members."
He defended Michal Kaminski, of Poland's Law and Justice party, pointing out that he had given a speech on the problem of anti-Semitism in Europe, and had visited Israel. He said: "These are not the habits of someone who is anti-Semitic".
He said of the Latvian party: "This is a respectable party in Latvia ... To suggest it is anti-Semitic or neo-Nazi is ludicrous."
The Washington state department refused to comment directly on the issue, in an attempt to avoid being accused of interfering in domestic UK politics.
The Obama administration favours a united EU, particularly over issues such as Iran. Hague tried to persuade Clinton that the Conservatives' Eurosceptic stance would not necessarily reduce Britain's role in Europe.
"I always put the case to all American leaders that it is important to distinguish between Europe's institutions and its capacity to act," he said. "I have explained to her and others that she will be find a Conservative government very active and activist from the very beginning in European affairs."
