President Joe Biden has again warned that peace in Northern Ireland must not be jeopardised as a result of complications caused by Brexit.
Mr Biden was speaking at the start of talks in Washington with Boris Johnson.
He said he had concerns about the Irish border, amid continuing issues with the Northern Ireland Protocol.
Mr Johnson said they were at one on the issue and that nobody wanted to see anything that would interrupt or imbalance the Good Friday Agreement.
The protocol keeps Northern Ireland aligned with the EU's single market for goods to ensure free trade across the Irish border.
However, it has led to additional checks on goods being traded across the Irish Sea between Great Britain and Northern Ireland, which has drawn criticism and protests from unionist politicians in Northern Ireland who have called for it to be scrapped.
The UK wants a fundamental renegotiation of the protocol but the EU has said that is not possible, although it is prepared to consider additional flexibilities.
Mr Biden told media at the White House that he felt "very strongly" about the protocol.
"We spent an enormous amount of time and effort, the United States, it was a major bipartisan effort made.
"And I would not at all like to see, nor I might add would many of my Republican colleagues like to see, a change in the Irish accords, the end result having a closed border in Ireland."
With what we have heard about the protocol in the last 24 hours, it doesn't feel that we are much further on - everyone has dug in to their positions.
Basically Joe Biden's position is what it has always been, that any outcome from Brexit that leads to a hardening of the Irish border would, in his view, do great damage to the Good Friday Agreement and therefore would do great damage to US-UK relations.
We have also had George Eustace, the UK Agriculture Minister, this morning suggesting that perhaps Joe Biden doesn't really understand the issues and that he's just been reading the Irish talking points.
Not necessarily great for the relationship between the two governments.
There may not be a lot of common ground on Brexit and Ireland between the two.
But we should be clear that the UK-US relationship is multi-stranded and around some areas, like climate change and on defence, the two sides are very close together.
Earlier this year, the president warned that the UK-EU dispute over border controls should not risk the peace process in Northern Ireland.
Reacting to Mr Biden's comments, Edwin Poots from the Democratic Unionist Party (DUP) said it was not a surprise.
"I think they need to understand what the Belfast (Good Friday) Agreement actually does say and go and read it and if they do that they will recognise that the Northern Ireland Protocol is actually damaging to the Belfast Agreement," he said.
"It is creating a border where there shouldn't be one and therefore the protocol has to go in order to meet the requirements of the Belfast Agreement.
"Washington aren't the decision makers here, the UK government will ultimately be the decision maker."
Northern Ireland's Finance Minister Conor Murphy, a Sinn Féin assembly member, said the US had maintained a "consistent position" in supporting the Good Friday Agreement.
He explained: "I'm glad that he reiterated that position, I'm sure it was food for thought for the British prime minister that the American administration and the EU, ourselves, most of the people here in the north, the Irish government are all on the same script in terms of protecting the Good Friday Agreement."
"We're going to talk a little bit about trade today and we're going to have to work that through," he said.
A UK government spokesperson said its priority was still getting a deal with the US alone.