US Secretary of State Mike Pompeo said his country would be ready "pen in hand" to sign a free-trade deal with the United Kingdom post-Brexit, but US legislators who would have to approve any such deal have made it clear things would not be so simple because of the issue of the Irish border.

The United States has a high number of people of Irish ancestry, who form an influential political bloc. One such politician, Congressman Brendan Boyle, is part of the committee that would need to approve such a deal. He said expectations of anything happening in a hurry were "fantasy land".

"The focus of the ways and means committee and (US trade representatives Robert) Lighthizer has been almost exclusively on China and USMCA (US-Mexico-Canada Agreement)," he said.

"Even in the best of circumstance, the idea that we would drop everything and move toward a deal with the UK, a market of 60 million people, is fantasy land."

Pompeo made his comments after meeting new British Foreign Secretary Dominic Raab, who along with Secretary of State for International Trade Liz Truss has been in Washington this week as the first representatives of British Prime Minister Boris Johnson's government to visit the US.

"We support the United Kingdom's sovereign choice, however Brexit ultimately shakes out," said Pompeo. "And we'll be on the doorstep - pen in hand - ready to sign a new free-trade agreement at the earliest possible time."

Particularly in the aftermath of a possible no-deal Brexit, economic ties with the US will be crucial for the UK, but currently Britain does not even have an ambassador in Washington.

The most recent holder of the office, Kim Darroch, resigned after a row over leaked diplomatic messages in which he criticized the Trump administration, drawing a furious response from Trump and a notable lack of support from Johnson, shortly before he became prime minister.

The status of the post-Brexit border between Northern Ireland, part of the UK, and the Republic of Ireland, an EU member state, remains a huge obstacle. The Good Friday Agreement of 1998, which ended three decades of political violence in Northern Ireland, involved significant input from US politicians including then-president Bill Clinton.

The potential reintroduction of a hard border between Northern Ireland and the Republic, which could be a consequence of Brexit, goes against its spirit, which is why the Irish-American lobby is so concerned.

But Raab insisted that even after a potential no-deal Brexit, the government would "do whatever it takes to avoid any hard border between the North and the Republic (of Ireland)".