DUP leader Jeffrey Donaldson has written to the Speaker of the US House of Representatives outlining his opposition to the Northern Ireland Protocol.
Mr Donaldson attached a copy of the Good Friday Agreement in his letter to Nancy Pelosi.
He told her the 1998 peace agreement was being "sacrificed" to facilitate the Irish Sea border created after Brexit.
Last week, Ms Pelosi said there could be no UK-US trade deal if the agreement were to be undermined by Brexit.
She was speaking in advance of a visit by British Prime minister Boris Johnston to the United States.
Mr Donaldson said the political, economic and constitutional difficulties created by the protocol threatened prosperity in Northern Ireland and its constitutional position within the United Kingdom.
"The Belfast Agreement was supposedly designed to protect all communities in Northern Ireland," he told Ms Pelosi.
"Not a single elected unionist in Northern Ireland supports the protocol yet your Office still champions it. How can this be?
"One either supports the principles of the Belfast Agreement or one supports the NI protocol, but it is not possible to sustain support for both."
He said the position being adopted by the EU was endangering the Good Friday Agreement which required unionist as well nationalist support.
Earlier the European Commission vice-president Maroš Šefčovič briefed European and foreign affairs ministers on his recent visit to Northern Ireland.
Mr Šefčovič will bring forward new EU legislation within the coming days which, diplomats have said, will address the problems surrounding the importation and licensing of medicines moving from Britain to Northern Ireland.
He will also introduce new proposals on customs formalities, agri-food checks and an enhanced role for the Northern Ireland institutions.
He said: "Solutions that we seek will be in the framework of the Protocol on Ireland / Northern Ireland. We are absolutely convinced that the Protocol is the best solution we found with the UK to address the unique situation of the island of Ireland.
"Because it means stability, certainty and predictability – and ultimately opportunities, as was confirmed to me during my meetings with Northern Irish stakeholders on the ground last week."
