The trip comes just days after Foreign Secretary Liz Truss caused worry in Washington after she announced the UK's intention to override parts of the Northern Ireland Protocol, reports Kathryn Samson
Nine of America's most senior lawmakers have travelled to the UK on a special mission - backed by President Joe Biden - to urge the UK government not to rip up parts of the Northern Ireland Protocol.
The group from the US - who see efforts to change the post-Brexit deal as a real danger to the Good Friday Agreement - met with Foreign Secretary Liz Truss, International Trade Secretary Anne-Marie Trevelyan and Labour leader Sir Keir Starmer.
Accounts of the talks so far have been thin on detail, with Ms Truss tweeting that the two sides "discussed our cast-iron commitment to the Belfast (Good Friday) Agreement, the importance of free trade and our condemnation of Russia's illegal invasion of Ukraine".
It follows a warning from US House speaker Nancy Pelosi that Congress will not support a free trade agreement with the UK if the government persists with “deeply concerning” plans to “unilaterally discard” the Northern Ireland Protocol, following comments made by Ms Truss.
What is the Northern Ireland Protocol?
The Northern Ireland Protocol, negotiated between Britain and the EU as part of the Brexit Withdrawal Agreement, allows goods to cross from the Republic of Ireland to Northern Ireland without checks.
There were no border checks between the two countries before Brexit, so the Protocol ensured this was maintained to respect the Good Friday Agreement - or the 1998 peace agreement.
However, the EU has strict rules on certain goods - like food - and requires checks before they are brought in to the block.
Therefore checks are made in England, Wales or Scotland before entering Northern Ireland, which has caused controversy and essentially created a border in the Irish sea.
This has made unionist parties in the country feel that they are growing apart from the UK.
In a strongly-worded intervention on Thursday, Ms Pelosi urged the UK and the EU to continue negotiations on the post-Brexit trade arrangements to uphold peace in the region.
The congresswoman said in a statement: “The Good Friday Accords are the bedrock of peace in Northern Ireland and a beacon of hope for the entire world.
“Ensuring there remains no physical border between the Irish Republic and Northern Ireland is absolutely necessary for upholding this landmark agreement, which has transformed Northern Ireland.
“It is deeply concerning that the United Kingdom now seeks to unilaterally discard the Northern Ireland Protocol, which preserves the important progress and stability forged by the Accords.”
The latest controversy has been sparked by Ms Truss’s announcement on Tuesday that the UK intends to legislate to override parts of the Brexit withdrawal treaty it struck with the EU.
The foreign secretary told the Commons the move is needed to reduce “unnecessary bureaucracy” and to protect the Good Friday Agreement, arguing that the EU’s proposals “would go backward from the situation we have today”.
The ongoing row over the treaty has created an impasse in efforts to form a devolved government administration in Belfast, with the Democratic Unionist Party (DUP) refusing to join an executive unless its concerns over the situation are addressed.
Unless the DUP agrees to form the Executive alongside Sinn Fein, the devolved government in Northern Ireland cannot get up and running.
Ms Pelosi’s intervention was met with scorn from former Brexit minister Lord Frost, who called the statement “ignorant” of the “the realities in Northern Ireland”.
“There is no plan to put in place a physical border,” he told the BBC.
“Nobody has ever suggested that, so I don’t know why she is suggesting that in her statement.”
DUP leader Sir Jeffrey Donaldson also described Ms Pelosi’s contribution as “entirely unhelpful”.
Ms Pelosi is not the only senior figure in Washington to express concern about relations between the UK and the EU in recent days.
Derek Chollet, a senior adviser to US Secretary of State Antony Blinken, said on Friday a “big fight” between the UK and the EU is the “last thing” the US wants.
Mr Neal told The Guardian part of his job is to convince the UK not to breach the Brexit treaty.
“They haven’t breached it yet. They’re talking about breaching it, so part of my job is to convince them not to breach it,” he said.
“My purpose is manifold but we really want to reaffirm America’s unwavering commitment to the Good Friday Agreement and to remind everybody that on the eve of the 25th anniversary of the Good Friday Agreement, it has worked splendidly.
“I want to remind everybody in the UK, in Northern Ireland that it should not be treated as a cavalier achievement.”
Ms Trevelyan said she was “delighted” to welcome a bipartisan congressional delegation led by top Democrat Richard Neal to her department to discuss UK-US trade matters, as well as the situation in Ukraine, but made no explicit mention of post-Brexit tensions.
A spokesperson for Sir Keir said his meeting featured talks on the need to protect the Good Friday Agreement by ensuring a working Northern Ireland Protocol.
The Labour leader and congressional delegation also touched on the need to be ambitious and creative in trade dialogues between the US and UK, and the importance of western unity in the face of Russian aggression in Europe, the spokesperson said.
The US delegation are to travel on to Belfast and Dublin for more talks.