- President-elect Joe Biden has said America does not want a "guarded border" between the Republic of Ireland and Northern Ireland following Brexit.
He said it had taken a lot of hard work to reach a settlement on the island of Ireland, ending decades of conflict.
The UK government has threatened to overrule the Brexit withdrawal agreement which committed it to keeping the border open.
Mr Biden has previously said Brexit must not endanger the peace process.
Speaking to reporters in Wilmington, Delaware, on Tuesday, Mr Biden said: "We do not want a guarded border. We've worked too long to get Ireland worked out.
"The idea of having a border north and south once again being closed is just not right, we've just got to keep the border open."
Mr Biden - who has unveiled officials for six important posts as he prepares to take office - said he had spoken to Prime Minister Boris Johnson, the Taoiseach (Irish prime minister) and the French government.
Mr Biden - an Irish American - has previously said that any UK-US post-Brexit trade deal had to be "contingent" on respect for the Good Friday Agreement .
Those comments came after the government put forward legislation that would give it the power to change aspects of the EU withdrawal agreement, a legally binding deal governing the terms of Brexit made earlier this year.
The Internal Market Bill is designed to enable goods and services to flow freely across England, Scotland, Wales and Northern Ireland after 1 January - when the post-Brexit transition period runs out.
The president-elect was quick to criticise the legislation when it was first published, warning that the Good Friday peace agreement in Northern Ireland "cannot become a casualty of Brexit".
Earlier this month, the House of Lords voted to remove a section of the bill that would allow ministers to break international law, but the government has said it would reinstate these clauses when the bill returns to the House of Commons next month.
The UK and EU are still working to strike a deal to govern their trading relationship once the UK's post-Brexit transition period ends in January 2021.
The President of the EU Commission, Ursula von der Leyen, said last week that progress had been made on some issues but there were still "some metres to the finish line" to reach agreement.
Meanwhile, a senior Democrat with close links to Mr Biden's incoming administration has criticised the UK government's plans to cut its overseas aid budget.
John Podesta, a former adviser to Presidents Obama and Clinton, told the BBC the reduction of the spending target was "extremely unfortunate".
Mr Podesta may not speak directly for Team Biden but his remarks will be seen as a clear warning shot from across the Atlantic, the BBC's diplomatic correspondent James Landale has said.
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