Joe Biden has said he will not allow peace in Northern Ireland to become a “casualty of Brexit” if he is elected US President in November.
The Democratic candidate said any UK-US trade deal had to be “contingent” on respect for the Good Friday Agreement.
Foreign Secretary Dominic Raab has been trying to reassure US politicians about the latest Brexit developments during a trip to Washington.
US Secretary of State Mike Pompeo said he trusted the UK to “get this right”.
But US Speaker Nancy Pelosi said there was “no chance” of a UK-US trade deal getting through the US Congress if the UK violated international agreements, undermining the Good Friday Agreement.
After a meeting with Mr Raab, the Speaker of the House of Representatives said the UK’s exit from the EU could not be allowed to “imperil” peace in Northern Ireland.
Brexit is high on the agenda at the Washington meetings, after the Internal Market Bill cleared its first parliamentary hurdle earlier this week.
The proposed law would give the UK government the power to override part of the Brexit withdrawal deal – which Prime Minister Boris Johnson agreed with the EU last October.
It now faces further scrutiny in the House of Commons, and also needs to be passed by the House of Lords.
If the law comes into force, it would breach international law – a prospect that prompted an angry response from senior figures in the US last week.