Rishi Sunak and Joe Biden have discussed the war in Ukraine during a phone call, as the US president called the United Kingdom his country's "closest ally".
The new prime minister, who took over from Liz Truss on Tuesday, will meet Biden in the coming weeks at the G20 summit in Indonesia.
A Downing Street spokesman on Tuesday said Biden congratulated Sunak on his appointment, adding that "the leaders looked forward to working closely together".
Only in recent days had Biden appeared to publicly criticise Truss' doomed economic strategy, in a rare intervention by president.
Relations between the two countries have also been somewhat strained in recent years amid ongoing UK-EU tensions over post-Brexit arrangements in Northern Ireland, with White House concerns over the impact on peace in the region.
The issue appears to have come up in the call, according to details released by Downing Street, as well as the war in Ukraine and the rising power of China.
The spokesman said: "President Biden congratulated the prime minister on his appointment and the leaders looked forward to working closely together. President Biden said that the UK remains America's closest ally, and the prime minister agreed on the huge strength of the relationship.
"The leaders discussed the extent of UK-US cooperation, both bilaterally and in regions such as the Indo-Pacific where the AUKUS pact forms part of our efforts to enhance stability and counter China's malign influence.
"They reflected on the leading role our countries are playing in supporting the people of Ukraine and ensuring Putin fails in this war.
"The prime minister and President Biden also agreed on the need to ensure the people of Northern Ireland have security and prosperity through preserving the Belfast (Good Friday) Agreement.
"The leaders looked forward to meeting in person at the upcoming G20 Summit in Indonesia."
Australian Associated Press
