Democrat Nancy Pelosi, whose party controls the House, said the UK’s exit from the EU could not be allowed to endanger the Irish peace deal.
Her comments came after the US national security adviser said the UK would be “first in line” for a trade deal.
John Bolton spoke after meeting Prime Minister Boris Johnson in London.
The reimposition of frontier controls between Northern Ireland and the Republic of Ireland if the UK leaves the EU without mutual agreement on 31 October – a so-called “hard Brexit” – is seen as a threat to the 1998 Good Friday Agreement, which ended decades of bloodshed in Northern Ireland.
“Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday Agreement, including the seamless border between the Irish Republic and Northern Ireland,” Ms Pelosi said in a statement on Wednesday.
‘Tough old haggle’
Mr Bolton said on Tuesday that the Trump administration supported a no-deal Brexit, and added Washington would propose an accelerated series of trade deals in the event of one.
He said these could be done on a “sector-by-sector” basis, with an agreement on manufacturing made first. A trade deal for financial services and agriculture would not be the first to be agreed, he added.
Asked whether his proposed plan would follow World Trade Organization (WTO) rules, Mr Bolton said “our trade negotiators seem to think it is”.
He said there would be enthusiastic bipartisan support in Congress for speedy ratification at each stage.
Mr Johnson said there were “all sorts” of opportunities for UK business in the US, particularly service companies, but the negotiations will be a “tough old haggle”.
However, critics warn that the UK will have to give in to some US demands in return for any trade agreement.
Former UK Foreign Secretary Jack Straw, who served under a Labour government, described Mr Bolton as “dangerously bellicose”.
He suggested the UK would have to agree to some US demands, for example allowing imports of US chlorine-washed chicken.
“This is a highly transactional administration… you don’t get something for nothing,” he told BBC Radio 4’s Today programme.
Lewis Lukens, a former deputy chief of mission at the US embassy in London and former acting US ambassador, said Mr Bolton was aligned to President Trump’s “America first agenda” and would be making “strong demands” on the UK to back the US position on issues like China, Iran and Chinese tech giant Huawei.
Mr Johnson is expected to have his first face-to-face meeting as prime minister with Mr Trump later this month at the G7 summit in France.