The US speaker Nancy Pelosi has described as "deeply concerning" moves by the UK to unilaterally scrap parts of the Northern Ireland protocol.
In remarks signalling further anticipated pressure from Washington on Boris Johnson's government, Ms Pelosi reiterated the US Congress would not support a free trade agreement with the UK in such circumstances.
“Ensuring there remains no physical border between the Irish Republic and Northern Ireland is absolutely necessary for upholding this landmark [Belfast] agreement, which has transformed Northern Ireland,” she said in a statement on Thursday.
"It is deeply concerning that the United Kingdom is now seeking to unilaterally discard the Northern Ireland protocol. Negotiated agreements like the protocol preserve the important progress and stability forged by the Good Friday Accords, which continue to enjoy strong bipartisan and bicameral support in the United States Congress."
The British government has already come under pressure from the US administration over its stance on the Border and trade in the aftermath of Brexit.
Ms Pelosi in her statement re-emphasised the message she has already made clear to Mr Johnson, the foreign secretary and members of the House of Commons, that "if the United Kingdom chooses to undermine the Good Friday Accords, the Congress cannot and will not support a bilateral free trade agreement with the United Kingdom".
“Respectful of the will of the British people and of Brexit, I urge constructive, collaborative and good-faith negotiations to implement an agreement that upholds peace,” she said.
“The children of Northern Ireland, who have never known the bloody conflict and do not want to go back, deserve a future free of the violence where all may reach their fulfilment.”