US President-elect Joe Biden has stressed the importance of protecting Northern Ireland's peace deal in the Brexit process during a call to UK Prime Minister Boris Johnson.
The message is being interpreted as hinting at potential tensions over Britain's EU exit even with the two leaders emphasising common ground in other areas.
Johnson's government is seeking a trade deal with the EU but is willing to leave without one, which could complicate a sensitive situation at the Northern Irish border with Ireland - the UK's only land border with the EU.
The 1998 Good Friday peace deal that effectively ended Northern Ireland's 30 years of sectarian violence created new institutions for cross-border co-operation on the island of Ireland.
But Johnson has put forward legislation that would break the Northern Ireland protocol of the Brexit divorce treaty that seeks to avoid a physical customs border between the British province and EU-member Ireland.
That prompted a warning two months ago from Biden, who has talked about the importance of his Irish heritage, that the UK must honour the 1998 agreement as it withdraws from the bloc or there can be no separate US trade deal.
"They talked about the importance of implementing Brexit in such a way that upholds the Good Friday Agreement," a British official said after Tuesday's call.
"The PM assured the president-elect that would be the case."
Johnson has predicted close ties with the United States under Biden, seeing common ground on issues like climate change.
"Among the shared priorities they discussed were containing COVID-19 and ... combating climate change," Biden's transition team said in a statement.
"The president-elect expressed his interest in co-operating with the UK, NATO and the EU on shared transAtlantic priorities and reaffirmed his support for the Good Friday Agreement in Northern Ireland," they added.
Johnson has never met Biden and commentators have suggested he will have to work hard to foster the so-called "special relationship" between the historical allies.