US President Joe Biden opened the first overseas trip of his term Wednesday, arriving in Britain with a declaration that the "United States is back.''

Biden, keen to repair relations with European allies that were shaken by his predecessor Donald Trump, is beginning his tour in the UK, where he was scheduled to meet Prime Minister Boris Johnson on Thursday.

What did Biden say?

Biden, who arrived at the Royal Air Force's Mildenhall air base in eastern England, addressed the first remarks of his maiden tour abroad to US troops stationed in the UK.

"This is my first overseas trip as president of the United States. I'm heading to the G7, then the NATO ministerial and then to meet with Mr. Putin to let him know what I want him to know," Biden said, drawing cheers from the crowd.

"At every point along the way, we're going to make it clear that the United States is back, and democracies are standing together to tackle the toughest challenges and issues that matter the most to our future," he said.

"That we're committed to leading with strength, defending our values, and delivering for our people."

Most pressing among the challenges facing the US and its allies is the coronavirus pandemic, with Biden promising that the US would also lead on this matter.

"We have to end COVID-19 not just at home — which we're doing — but everywhere," Biden said.

According to US media reports, the Biden administration is set to buy 500 million doses of the Pfizer-BioNTech vaccine for international distribution.

'Rock-solid belief' in Belfast Agreement

On Thursday, Biden was expected to underline the US insistence on safeguarding the 1998 Belfast Agreement (also known as the Good Friday Agreement) in his meeting with the UK prime minister.

Biden's staff said the president had been clear, ahead of their first official meeting, that Brexit-related issues should not be allowed to derail the US-brokered agreement.

"President Biden has been crystal clear about his rock-solid belief in the Good Friday Agreement as the foundation for peaceful co-existence in Northern Ireland," Biden's national security adviser Jake Sullivan told reporters aboard Air Force One.

"Any steps that imperil it or undermine it would not be welcomed by the United States," said Sullivan, who declined to elaborate on issues surrounding Brexit.

Sullivan stressed that Biden, who is known to be proud of his Irish heritage, would make a statement of principle about the importance of that peace deal.

"He's not issuing threats or ultimatums, he's going to simply convey his deep-seated belief that we need to stand behind and protect this protocol," Sullivan said.

Northern Ireland's customs conundrum

The issue of border customs checks — and particularly where they should take place — has stoked sectarian divisions, and rekindled fears of a return to violence.

While the 1998 deal ended decades of violence between mainly Protestant pro-UK unionists and largely Catholic Irish nationalists, the province remains deeply divided.

Ahead of Brexit, the EU and Britain had tried to solve the border riddle presented by the need for a customs border. However, London has since said that the protocol it signed with Brussels is unsustainable.

The EU has already launched legal proceedings after the UK delayed promised custom controls on some goods arriving in Northern Ireland from Great Britain — England, Scotland and Wales.

rc/msh (AFP, AP, dpa, Reuters)