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.
UK Foreign Secretary Dominic Raab has been trying to reassure US politicians about the latest Brexit developments during a trip to Washington, BBC reported.
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.