A senior US politician has warned that lawmakers could block a future US-UK trade deal if the Good Friday Agreement is undermined.
Senate Democratic leader Chuck Schumer has written to US Secretary of State Mike Pompeo warning that Congress could work on a cross-party basis to block a deal if a hard border is introduced on the island of Ireland.
A copy of the letter was also sent to UK Foreign Secretary Dominic Raab.
Mr Schumer called for the Trump administration to stop "over-promising an unconditional and unrealistic" post-Brexit trade agreement with the United Kingdom.
He added he will work with Democratic colleague Nancy Pelosi, the speaker of the House of Representatives - and Republicans - to block any deal that threatens the Belfast Agreement.
In his letter, Mr Schumer said: "While Britain is a unique and valued ally of our nation, as the Democratic Leader of the United States Senate, which would consider prospective new bilateral trade agreements, I write to express my inveterate opposition to any prospective trade deal with UK that either undermines the landmark Good Friday Agreement or facilitates a return to a hard border.
"As we approach the fiftieth anniversary of the traumatic events that precipitated the long and difficult period known as 'The Troubles', all stakeholders would do well to reflect on the hate, violence, injustice, lawlessness and societal upheaval of that time - and of the extraordinary transformation ushered in by the Good Friday Agreement.
"Indeed, this policy is broadly and deeply supported in the Irish-American community, which is keenly following the coming resolution of the UK's Brexit decision."
He added: "The free and demilitarised border on the island of Ireland between the Republic of Ireland and Northern Ireland is one of the precious products of that framework.
"Currently, upwards of 35,000 people commute daily over this now-invisible line that was once covered with razor wire and armoured cars. This radical change has unleashed significant economic energy and facilitated deep societal interconnection.
"It is not surprising, then, that 56% of the people of Northern Ireland voted against Brexit.
"The Good Friday Agreement is a towering achievement of diplomacy and it planted the seeds of a society based on mutual respect and equality, rather than one based on distrust and discrimination."
Mr Schumer continued: "The notion that America would now endorse a policy or agreement that undermines the success of the Good Friday Agreement is profoundly counterproductive and risks exacerbating sectarian polarisation and eroding self-determination - and unleashing the potential for violence that comes with that reality. This cannot be allowed to happen.
"Plainly stated, America should not be in the business of handing out a blank cheque that bankrupts the peace, security, self-determination and shared prosperity precipitated by the Good Friday Agreement.
"America's policy should be to realise the full potential of the Good Friday Agreement, not to erode it or entertain the possibility of a return of a hard border or direct rule."