Secretary of State Rex Tillerson defended the "special relationship" between Britain and the United States during a visit to London on Monday, following a series of spats since the election of Donald Trump that have strained ties.
Tillerson paid a discreet visit to the new US embassy in London after Trump cancelled plans to open it himself -- the latest in a spate of damaging rows.
The top US diplomat later met with counterpart Boris Johnson and Prime Minister Theresa May, speaking of the need to reset the relationship between the historic allies.
"We spend a lot of time talking about the world's problems... and sometimes we forget about the importance of our own relationship," Tillerson said as he met Johnson.
"We need to pay attention to that relationship... we treasure it," he added.
Johnson described the relationship as "absolutely fundamental" to Britain's economy.
The pair talked about Yemen and Turkey's assault on Kurdish militia in Syria.
Tillerson also met with May in her Downing Street office, where they discussed the Iran nuclear agreement.
"They agreed on the importance of the international community coming together to counter Iran's destabilising regional activity, and the prime minister reiterated the UK's commitment to the Iran nuclear deal," Downing Street said in a statement.
Tillerson earlier bucked tradition by not holding a formal meet-and-greet during his visit to the new embassy, which the State Department said was due to the current shutdown of the US government over a budget impasse in Congress.
But even before the shutdown, the State Department had already played down Tillerson's visit to the embassy, which opened last week, due to controversy over Trump's refusal to inaugurate the new building.
The president said he was unhappy at the cost and location of the new embassy, a futuristic cuboid building surrounded by a moat.
But the decision came after a series of rows and after it became clear the visit would be met with mass protests.
- 'Bad deal' -
Tillerson posed for photographs before being shown around the new embassy.
Trump had been due to inaugurate the building in February, but cancelled the visit, tweeting: "having sold perhaps the best located and finest embassy in London for 'peanuts,' only to build a new one in an off location for 1.2 billion dollars. Bad deal. Wanted me to cut ribbon-NO!"
The new building is slightly outside central London in an area south of the River Thames which is being regenerated, unlike its predecessor, which was in a plush quarter in the heart of the British capital.
US ambassador Woody Johnson brushed off Trump's criticism of the building, telling reporters: "The embassy actually is going to really work."
When asked whether there would be a ribbon-cutting ceremony, the ambassador added: "At some point we're going to do it, but there's no urgency to that. We'll do it when the time is right."
Trump has yet to visit Britain since taking office a year ago, and has been involved in rows with the government over issues including trade and his retweeting of a video posted by a British far-right group.
London Mayor Sadiq Khan also said he "would not be welcome" in the city, suggesting there could be widespread protests.
Boris Johnson wrote in The Sunday Telegraph newspaper that Trump should be welcomed to Britain because the country's ties with the US are vital for security and provide the UK's "single most extraordinary economic relationship".
Trump "is the elected president of the world's most powerful democracy" and resisting his visit to the UK risks "damaging the national interest", Johnson added.
May is due to meet with Trump at the World Economic Forum in Davos late this week, but the plans could be thrown into disarray by the US shutdown.
Tillerson's mini-tour of Europe will continue with a visit to Paris on Tuesday.