The British government has struggled to overcome American opposition to its plan to breach the Brexit withdrawal agreement with the European Union after Democratic presidential candidate Joe Biden became the latest US politician to express alarm.
Prime Minister Boris Johnson's move to break parts of the EU divorce deal relating to Northern Ireland has triggered fears it could undermine the 1998 Good Friday peace accord that ended decades of violence between Irish nationalists and British unionists.
"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," Biden tweeted.
"Any trade deal between the US and UK must be contingent upon respect for the agreement and preventing the return of a hard border. Period."
Britain and the EU jointly promised in the Brexit agreement to ensure there are no customs posts or other obstacles on the Northern Ireland-Ireland border. The open border is key to the stability that underpins the peace settlement.
British Foreign Secretary Dominic Raab is in Washington this week and has been trying to assuage American concerns that a pending government bill would undermine Northern Ireland peace, if passed by MPs.
He is meeting US politicians including House Speaker Nancy Pelosi, who has warned Britain will not secure a much sought-after trade deal with the United States if it undermines the peace accord.
Raab insists the UK has an "absolute" commitment to the Good Friday Agreement. He described Britain's planned law as "precautionary" and "proportionate".
Johnson argues the law is intended to be an insurance policy against unreasonable behaviour by the European Union that could threaten the unity of the UK by disrupting trade between Northern Ireland and the rest of the UK.
The bloc has demanded the UK drop the plan by the end of September or face legal action.
Johnson told The Sun the EU was being "abusive" to Britain and risking four decades of partnership.
He said the UK must "ring-fence" the Brexit deal "to put in watertight bulkheads that will stop friends and partners making abusive or extreme interpretations of the provisions."
Five former British prime ministers have criticised Johnson's willingness to break international law and the government's top legal civil servant and most senior law officer for Scotland have resigned.
In an attempt to quiet unease among MPs from Johnson's Conservative Party, the government on Wednesday offered a compromise that would require parliament to vote before the government took any actions that broke international law.
It came as EU Brexit negotiator Michel Barnier told the bloc's 27 national envoys he still hoped a trade deal with Britain was possible, stressing the coming days would be decisive, diplomatic sources told Reuters.
The UK withdrew from the EU's political institutions on January 31 but remains in a tariff-free transition period until the end of the year while negotiators work out the terms of a future trade relationship.