The European Union last week said it was ready to start talks on a trade agreement with the United States and aims to conclude a deal before year-end.
"If America wants a trade agreement with the European Union, which I think is very desirable – I want it – at the same time you are back to the same issue on the border if you do anything that dampens or softens the Good Friday Agreement," Democratic Congressman Richard Neal was quoted as saying.
Neal is visiting Ireland with U.S. House of Representatives Speaker Nancy Pelosi, who on Wednesday said the United States would also not agree to any trade deal with Britain if future Brexit arrangements undermine peace in Ireland, reiterating comments made by the congressman in February.
The European Union has insisted it will not accept any British withdrawal agreement that results in any infrastructure on the border between Northern Ireland and Ireland, something that would anger Irish nationalists and could become a target for militants.
But some British politicians have called on Brussels to soften this demand to get a deal done.
Neal, chairman of the Congressional committee overseeing trade, said any Brexit deal must maintain the sanctity of the peace agreement, the Irish Times reported.
How to keep EU-member Ireland's 500km (350 mile) border with Northern Ireland open after Brexit is proving the most intractable issue in Britain's tortuous efforts to leave the EU.
British Prime Minister Theresa May's government is in talks with the opposition Labour Party to build support for a Brexit divorce deal that parliament has already rejected three times, potentially delaying the UK's departure date from the European Union until the end of October.
Much of the opposition to May's deal within her own party is centred on fears that it would not provide a clean enough break to allow the United Kingdom to forge new trade deals around the world, especially with the United States.
(Reporting by Conor Humphries; Editing by Kirsten Donovan)