Joe Biden has stressed the importance of upholding the Northern Ireland Protocol in his first phone call with the new Prime Minister Liz Truss.
The transatlantic relationship could be strained if Ms Truss pushes ahead with the plan to override parts of the Protocol, with Mr Biden proud of his Irish roots and taking a keen interest in the issue.
According to the White House's account of the call, the two discussed their "shared commitment to protecting the gains of the Belfast/Good Friday Agreement and the importance of reaching a negotiated agreement with the European Union on the Northern Ireland Protocol".
The White House said the leaders also "discussed the importance of continued close cooperation on global challenges, including supporting Ukraine as it defends itself against Russian aggression, addressing the challenges posed by China, preventing Iran from ever acquiring a nuclear weapon, and securing sustainable and affordable energy resources".
A Downing Street spokesman said the pair reflected on the "enduring strength" of the special relationship between the UK and US.
"The Prime Minister looked forward to working closely with President Biden as leaders of free democracies to tackle shared challenges, particularly the extreme economic problems unleashed by Putin's war," she said.
"The Prime Minister and President Biden reflected on the enduring strength of the special relationship. They agreed that the partnership between our countries, fortified by our shared values, has been crucial for defending and advancing freedom and democracy in the world.
"They agreed to build on those links, including by furthering our deep defence alliance through Nato and Aukus."
Earlier, Mr Biden was the first world leader to congratulate Ms Truss on her new position.
Just as Ms Truss finished her speech and formally entered Downing Street, the US president tweeted: "Congratulations to Prime Minister Liz Truss.
"I look forward to deepening the special relationship between our countries and working in close cooperation on global challenges, including continued support for Ukraine as it defends itself against Russian aggression."
Ms Truss also accepted an invitation to visit Ukraine from President Volodymyr Zelensky during her first phone call as Prime Minister with a foreign leader.
In a statement, the Ukrainian president said he had thanked the British people for their support.
Mr Zelensky tweeted: "I became the 1st foreign leader to have a conversation with the newly elected (British) PM @trussliz. Invited her to (Ukraine). Thanked (British) people for the major defense & economic aid for (Ukraine). It's important that (Britain) is ready to further strengthen it. Attention was paid to security guarantees.
"We discussed the participation of (Britain) in the recovery of (Ukraine). Coordinated further pressure on the RF (Russian Federation). The goal is to stop the aggression & bring the perpetrators to justice. It's important to designate the RF a terrorist state. We'll continue active interaction in all formats."
A Downing Street spokesman said that Ms Truss had told the Ukrainian president that his country could count on the UK's assistance "for the long term".
"President Zelensky thanked the Prime Minister for the UK's staunch support and congratulated her on her appointment," she said.
"The leaders discussed the need to strengthen global security and the measures necessary to cut off the funds fuelling Putin's war machine.
"The leaders deplored Putin's attempts to weaponise energy, and the Prime Minister said it was vital Russia's blackmail did not deter the west from ensuring Putin fails. She also underscored the importance of ensuring the UK and our allies continue to build energy independence."
The spokesman added that the Prime Minister was "delighted" to accept an invitation to visit Ukraine.
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