Washington has strongly criticised Britain's decision to talk to the political wing of Hezbollah, and complained that Barack Obama's new administration was not properly informed of the move in advance.
The objection underlined the awkward start to the "special relationship" between the US and Britain after Gordon Brown's visit to Mr Obama last week.
The trip was beset by fears that the old alliance would wane under the new leadership in the White House and a flap over insufficiently thoughtful presents given by Mr Obama to the Prime Minister.
But the first serious policy disagreement has emerged over Hezbollah, the Lebanese movement which Washington brands as a terrorist organisation, following the Foreign Office's announcement last week that it would talk to the political branch of the militant group.
A senior US government official said that he would like to ask the British to "explain the difference between the political, military and social wings of Hezbollah", adding, "we don't see a difference between the integrated leadership that they see".
He noted that Hezbollah's strongholds in south Beirut had recently marked the first anniversary of the death of Imad Mughniyeh with mass celebrations. Mughniyeh was a senior figure in the organisation associated with the Beirut barracks and US embassy bombings in 1983, which killed more than 350 people, as well as the kidnapping of dozens of foreigners in Lebanon in the 1980s.
"For years Hezbollah denied knowledge of Imad Mughniyeh," said the official. "And now all over south Beirut are these posters extolling the virtues of Imad Mughniyeh."
Asked if Washington had been informed of the decision in advance by the Foreign Office, he said "informed under a previous administration" would be a "more accurate description".
At the time of its announcement, the Foreign Office issued a statement saying that Britain had "reconsidered our position on no contact with Hezbollah in light of more positive recent political developments in Lebanon", referring to the formation of a national unity government that included Hezbollah.
Washington has always maintained that Hezbollah is funded by Iran and has a long track record of terrorist activity. Though it has a political office and runs a network of welfare offices, the Americans take the view that it is one cohesive organisation.
The state department has publicly said that it was "alerted" that the British "were considering taking this position".
"We are not ready to take the same step. Our position on Hezbollah has not changed," said state department spokesman Gordon Duguid last week.
Ironically the Foreign Office may have been encouraged by the Obama administration's new openness, which has already seen Secretary of State Hillary Clinton dispatch two envoys to Syria for lengthy talks.
The US official said those talks had been "comprehensive and constructive", with both parties discussing mutual concerns and searching for common ground.