British Prime Minister Boris Johnson hailed US President Joe Biden as "a big breath of fresh air" on Thursday because he wanted to work with London on a huge range of issues ranging from climate change and COVID-19 to security.

"It's a big breath of fresh air," Johnson said of the talks with Biden. "It's new, it's interesting and we're working very hard together. We went on for about an hour and 20 or so. It was a long, long, good session. We covered a huge range of subjects."

Biden is keen to try to use the trip to burnish his multilateral credentials after the tumult of Donald Trump's presidency, which left many US allies in Europe and Asia bewildered and some alienated.

Johnson played down differences with Washington over the impact of Brexit on the peace in Northern Ireland, saying that the United States, United Kingdom and European Union had "complete harmony" on finding solutions to uphold the 1998 peace deal.

"There's complete harmony on the need to keep going, find solutions, and make sure we uphold the Belfast Good Friday Agreement," Johnson said. "And I think what's interesting is Northern Ireland is a fantastic place and it's got amazing potential. It is a great, great part of the UK."

Asked if Biden had made his alarm about the situation in Northern Ireland very clear, Johnson said: "No he didn't."

"America, the United States, Washington, the UK, plus the European Union have one thing we absolutely all want to do," Johnson said. "And that is to uphold the Belfast Good Friday Agreement, and make sure we keep the balance of the peace process going. That is absolutely common ground," Johnson said.

"You know, I'm optimistic."