London (AFP) - US President Donald Trump on Tuesday promised to "stay out" of Britain's election campaign during a two-day visit, while also seeking to defuse a key attack line against British Prime Minister Boris Johnson over healthcare.

Downing Street is braced for fireworks as Trump attends a meeting of the NATO military alliance in Britain, just days before a crucial election on December 12.

In his first press conference on Tuesday, the notoriously unpredictable US leader tried to avoid controversy in repeated questions about British politics.

"I'll stay out of the election," Trump said, adding: "I don't want to complicate it."

He declined to repeat his previous criticism of main opposition Labour leader Jeremy Corbyn, although he praised Johnson as "very capable and I think he will do a good job".

But answering questions during talks with NATO chief Jens Stoltenberg, he could not stay out of an election issue that Labour has put at the heart of its campaign.

Corbyn claims US firms are seeking to exploit Britain's state-run National Health Service (NHS) in any UK-US trade deal London signs after leaving the European Union.

Trump fuelled the row on his state visit to Britain earlier this year by stating that "everything" would be on the table in any post-Brexit trade talks.

But Johnson said last week that if Washington tried to include the NHS in trade talks, "we'd walk away".

On Tuesday Trump insisted: "We have absolutely nothing to do with it and we wouldn't want to if you handed it to us on a silver platter."

Protests planned

The president confirmed he would be meeting Johnson, who is co-hosting the NATO summit, but it is not clear if the pair will have formal one-to-one talks.

There is speculation that, in the middle of an election campaign, Johnson wants to avoid appearing too close to a US president who is deeply unpopular in Britain.

Several hundred protests marched from Trafalgar Square towards Buckingham Palace, where Queen Elizabeth II held a reception for Trump and other NATO leaders.

Opinion polls suggest Johnson's Conservative party is heading for a comfortable win in next week's election, which has been dominated by Britain's looming EU exit.

Trump supports Brexit but he publicly criticised former prime minister Theresa May's approach, suggesting she give up on trying to get a divorce deal with Brussels.

The US leader has since questioned whether Johnson's own deal would allow a trade deal with Washington -- something the premier touts as a big prize of Brexit.

Many protesters, including doctors and nurses, held up signs saying "Save Our NHS" and "Dump Trump". Police kept them at a distance from the palace until the leaders had gone inside.

"We don't believe Trump when he says the NHS is not on the table, because he's a liar," said Sarah Boston, 74, from west London.

The visitors were greeted by the queen's eldest son and heir, Prince Charles, and his wife, Camilla.

Security concerns

The US president has previously criticised Corbyn as "bad" for Britain, but he declined to repeat this on Tuesday.

In an intervention timed for the NATO summit, Johnson accused the Labour leader of being a security threat, saying that "every time he has the chance, he sides with our enemies".

He told The Sun tabloid that Britain's intelligence allies -- including the United States -- were "very anxious" about working with a Corbyn-led government.

Corbyn has a long history of criticising British and US military interventions, and has been accused of sympathising with proscribed terrorist organisations, from the IRA to Hamas.

The Labour leader described the claims, which come just days after a terror attack in London in which two people were fatally stabbed, as "complete nonsense".

And Trump said: "I can work with anybody, I'm a very easy person to work with."

Pressed further, he said: "I know nothing about the gentleman, really, know nothing about Jeremy Corbyn."

Corbyn boycotted a royal dinner during Trump's state visit to Britain but is due to attend the palace reception hosted by Queen Elizabeth II.

The NATO leaders will hold their substantive discussions on Wednesday near Watford, north of the capital.