Mr Biden will meet UK Prime Minister Boris Johnson in Cornwall on Thursday ahead of the G7 summit and is expected to deliver a warning not to let the Brexit trade dispute "imperil" the Northern Ireland peace process.

The president will also seek to show the United States is back as a major international diplomatic force following the Donald Trump years, using the visit to Europe to shore up transatlantic alliances.

Mr Biden flew in to the UK on Air Force One, landing at the US airbase at RAF Mildenhall before heading on to Cornwall for Thursday's talks with Mr Johnson and the G7 summit in Carbis Bay.

He told an audience of US military personnel and service families: "We are going to make it clear that the United States is back and democracies of the world are standing together to tackle the toughest challenges and the issues that matter most to our future."

He called for international action to tackle climate change and end the COVID-19 pandemic.

"This diplomacy is essential because no single nation acting alone can meet all the challenges we face today because the world is changing," Mr Biden said.

The US is expected to buy 500 million more doses of the Pfizer COVID-19 vaccine to share through the global Covax alliance for donation to 92 lower income countries and the African Union over the next year.

Ahead of the talks with Mr Johnson, Mr Biden's national security adviser Jake Sullivan warned the president harbours "very deep" concerns on the issue provoked by Brexit.

Mr Sullivan said the president believes the post-Brexit Northern Ireland Protocol is "critical" to ensuring that the Good Friday Agreement is protected, as Britain and the EU try to resolve the issue of checks on goods crossing the Irish Sea.

The adviser told the BBC that both sides must continue with negotiations, adding: "Whatever way they find to proceed must, at its core, fundamentally protect the gains of the Good Friday Agreement and not imperil that.

"That is the message that President Biden will send when he is in Cornwall."

Mr Sullivan declined to say whether a failure to alleviate the Northern Ireland concerns could jeopardise a transatlantic trade deal but added that "our concern runs very deep on the Northern Ireland issue".

The prime minister said he is "not worried" about the prospect of the summit being overshadowed by the row with the EU.

Asked about Mr Sullivan's warning, Mr Johnson told broadcasters in Cornwall that he was "very, very optimistic" about the situation and said a solution is "easily doable".

After touching down at RAF Mildenhall and addressing the personnel, Mr Biden and his wife Jill flew on to Cornwall.

Mr Biden's trip to Europe also takes in a Nato meeting and talks with Russia's Vladimir Putin "to let him know what I want him to know".

The prime minister plans to use the summit of leading economies to urge the G7 members - also including Canada, Japan, France, Germany and Italy - to "defeat" COVID-19 by helping to vaccinate the world by the end of next year.

Mr Johnson also wants a new treaty on responding to pandemics after the "pretty scratchy period" where countries were competing and "squabbling" over access to personal protective equipment.

The summit would focus on "building back greener, building back better" after the pandemic, he said.

After the summit ends, the president and first lady will meet the Queen at Windsor Castle.

Australian Associated Press