The Speaker of the United States House of Representatives has reiterated that Britain's exit from the European Union cannot be allowed to imperil the Good Friday Agreement.
Nancy Pelosi also said this had to include a seamless border on the island of Ireland.
She released a statement after US National Security Advisor John Bolton said the UK would be "front of the trade queue" for a new trade agreement with the US after Brexit.
Ms Pelosi said the peace of the Good Friday Agreement is "treasured by the American people" and "will be fiercely defended on a bicameral and bipartisan basis in the US Congress."
She said: "The Good Friday Agreement serves as the bedrock of peace in Northern Ireland and as a beacon of hope for the entire world.
"After centuries of conflict and bloodshed, the world has witnessed a miracle of reconciliation and progress made possible because of this transformative accord.
"Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday Agreement, including the seamless border between the Irish Republic and Northern Ireland, especially now, as the first generation born into the hope of Good Friday 21 years ago comes into adulthood.
"We cannot go back."
"If Brexit undermines the Good Friday accord, there will be no chance of a US-UK trade agreement passing the Congress.
"The peace of the Good Friday Agreement is treasured by the American people and will be fiercely defended on a bicameral and bipartisan basis in the United States Congress".
The remarks echo a speech made to the Dáil back in April, in which she said progress with the Northern Ireland peace process cannot be undermined by Brexit.
At the time, she said: "We treasure the Good Friday accord - not only because of what it has meant for Northern Ireland and for Ireland, that would be reason enough.
"We treasure the Good Friday accord because it is not just a treaty, it is an epic, it is a value, it is an article of faith for us - it is a beacon to the world.
"We treasure the Good Friday accord because of what it says is possible for the entire world, a reason to hope that in every place that dreams, that reconciliation will be possible for them too".
"I've said it before and I'll say it again: we must ensure that nothing happens in the Brexit discussions that imperil the Good Friday accord, including but not limited to the seamless border between the Irish Republic and Northern Ireland.
"Let me be clear: if the Brexit deal undermines the Good Friday accords, there would be no chance of a US-UK trade agreement.
"I say that hopefully that we would not have to face that reality - but I say it as a prediction."