Joe Biden has said he does not want to see a guarded border between Ireland and Northern Ireland.
Speaking to reporters in Wilmington, Delaware, the US President-elect said he had discussed the issue with Boris Johnson and other European leaders.
Mr Biden has previously intervened in the Brexit debate on borders, warning a breach of the Good Friday Agreement would threaten the prospect of a future US-UK trade deal.
“We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit,” he said in September.
“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.”
Please use Chrome browser for a more accessible video player
On Tuesday, Mr Biden also said he was yet to begin receiving the president’s daily intelligence reports, but confirmed he had been offered them.
The bulletin, called the president’s daily brief, consists of a collection of classified intelligence reports prepared every day to give the US leader updates on major security threats around the world.
More from Joe Biden
No mea culpa for all the bad blood, but Trump has bowed to public pressure
Trump tells his team to cooperate with transition but vows to keep up fight
Brexit: EU thanks Joe Biden for ‘clear support’ amid dispute over UK’s Internal Market Bill
John Kerry appointed to key Joe Biden cabinet role – find out who his picks are and why they matter
US election results: Judge rejects Trump’s ‘speculative’ bid to stop Pennsylvania vote certification
US election results: Georgia recount upholds Biden’s victory as Trump loses clutch of lawsuits
It comes as another sign Donald Trump is backing away from contesting the transition of power to Mr Biden – and is a day after he told his team to co-operate with the incoming administration.
He said on Monday that he had given the green light to the head of the General Services Administration (GSA) to proceed with the changeover.
The GSA is responsible for many of the basic services that allow the US government to function, from buildings and transport management to IT, financial services, supply chains and human resources.
Despite this turnaround, however, the incumbent has still vowed to keep up the fight against the election, having repeatedly refused to accept and concede that he lost.
Mr Trump has claimed widespread voter fraud skewed the final result earlier this month but has failed to ever produce evidence.
Earlier on Tuesday, the president-elect began announcing the first of his cabinet picks, including several firsts for America.
Avril Haines was introduced as the first woman to hold the position of director of national intelligence, while Alejandro Mayorkas, a Cuban-American, will be the first Latino and immigrant to lead the Department of Homeland Security.
Joe Biden’s cabinet picks – who they are and why they matter
John Kerry, the former secretary of state, has been appointed as the country’s first ever climate envoy – a full-time role focused solely on tackling climate change.
Other appointments include Anthony Blinken for secretary of state, Linda Thomas Greenfield as the US ambassador to the UN, and Jake Sullivan as national security adviser.
“It’s a team that reflects the fact that America is back, ready to lead the world, not retreat from it, once again sit at the head of the table, ready to confront our adversaries and not reject our allies, ready to stand up for our values,” Mr Biden said during his announcement.
He then pledged to ditch the “America first” strategy pursued by his predecessor, and instead move closer to working with US allies.
“Let’s begin that work,” he said, adding: “To heal and unite America as well as the world.”