US President Donald Trump kicks off a two-day whirlwind of meetings with Nato alliance members, but his focus appears to remain centered on the impeachment inquiry playing out at home.

Before departing for London to meet on Tuesday with other leaders from the 29-member alliance, Trump accused Democrats of trying to embarrass him by scheduling this week's impeachment hearing while he'll be abroad with Nato leaders.

Trump, who arrived in London late on Monday, called the trip one of the most important journeys that we make as president before he departed Washington and said Democrats had long known about it.

He was back to lashing out at Democrats minutes after landing in London late Monday night, touting a Republican impeachment report calling Trumps decision to hold up military aid to Ukraine entirely prudent.

Democrats contend that Trump abused presidential powers by holding up the aid to pressure Ukraine President Volodymyr Zelenskiy to investigate former vice president Joe Biden and his son.

"Prior to landing I read the Republicans Report on the Impeachment Hoax. Great job! Radical Left has NO CASE," Trump tweeted. "Read the Transcripts. Shouldn't even be allowed. Can we go to Supreme Court to stop?"

Trump's trip comes amid ongoing quarrels over defence spending by Nato allies and widespread anxiety over the American president's commitment to the alliance.

He has a busy schedule over the two days at the leaders conference, including scheduled one-on-one meetings with French President Emmanuel Macron, German Chancellor Angela Merkel, Canadian Prime Minister Justin Trudeau, Danish Prime Minister Mette Frederiksen and Italian Premier Giuseppe Conte.

He will also attend tea with Prince Charles and his wife Camilla, the Duchess of Cornwall. Queen Elizabeth II and British Prime Minister Boris Johnson will host Trump and the other Nato leaders at back-to-back receptions Tuesday evening.

Trump is also scheduled to squeeze in a campaign fundraiser with American expatriate supporters that is expected to raise $3 million for his re-election effort and the Republican National Committee.

Trumps meeting with Macron on Tuesday comes after Robert Lighthizer, the chief US trade representative, recommended $2.4 billion in new tariffs on French cheese, sparkling wine and other products.

The call for new duties came after his office completed a five-month investigation that concluded that a French digital services tax unfairly discriminates against US tech companies, including Google, Apple, Facebook and Amazon.

The president said his trip would be focused on fighting for the American people. But in the more than two months that the impeachment inquiry has been underway, he has constantly drifted back to what he frames as the Democrats unfair effort to overturn the results of his 2016 election.

The House Judiciary Committee is scheduled to hold a hearing Wednesday on the constitutional grounds for impeachment before Trump wraps up at the Nato meeting.

Trump, Secretary of State Mike Pompeo, White House counsel Pat Cipollone and presidential counselor Kellyanne Conway all complained about the timing, with Pompeo saying the hearings would distract the president from his important mission overseas.

Trump insists he is solely focused on scoring domestic and foreign policy wins, including revamping Nato so that allies spend more on defence. But he's often appeared consumed by the day-to-day battle against impeachment.

In recent days he has repeatedly lashed out about the impeachment hoax and the scam inquiry, even delving into impeachment at a ceremony to celebrate National Collegiate Athletic Association athletes and at last week's annual turkey pardon.

White House aides say the summit offers Trump an opportunity to counter the impeachment narrative in Washington and demonstrate to voters that he is keeping a business-as-usual approach while Democrats concentrate on the probe.

But soon after Air Force One departed, Trump took to Twitter to slam 'Do Nothing Democrats' for scheduling the hearing during the Nato meeting as 'Not nice!'.

House Speaker Nancy Pelosi, in Madrid for a United Nations conference on climate change, declined to comment on the impeachment inquiry: When we travel abroad, we don't talk about the president in a negative way. We save that for home.

Trump is only the fourth US president in history to face an impeachment inquiry. The gravity of impeachment is likely to play into the calculus of how other global leaders engage the president going forward, in the view of some analysts.

In one sense impeachment is weakening his hand diplomatically, said Ted Galen Carpenter, a senior fellow for defence and foreign policy studies at the Cato Institute in Washington. For a normal president, it would be seen as a substantial problem. For Donald Trump, he is going to try to blow right through it and act is if that's not a relevant factor.

The Nato leaders meeting is a complicated backdrop for Trump to make his first extended overseas visit. He made a quick Thanksgiving visit to US troops in Afghanistan since Democrats launched the impeachment inquiry.

Trump has repeatedly criticised fellow Nato members and complained that too few nations are on track to meet the alliance goal of spending at least 2 per cent of the gross domestic product on defence by 2024. Macron recently lamented that a lack of US leadership was causing the brain death of the alliance.

Trumps former national security adviser John Bolton has said the president could move to leave the alliance if he wins re-election.

Biden, who has warned that a second Trump term could mean Nato's effective dissolution, jokingly said Monday that he'd “say a prayer” as the president heads to London.

“There's time for redemption,” Biden told reporters as he campaigned in Emmetsburg, Iowa. “But so far he's treated Nato like it's a protection racket.” In the lead-up to the summit, White House aides sought to soften Trump's past criticism of member nations as deadbeats.

White House officials noted that before Trump took office only four Nato members had reached the 2% benchmark set in 2014. Now nine countries have reached the threshold, according to the White House. Eighteen of the 29 members of the alliance are projected to meet the 2% threshold by 2024.

Trump is not the first US president to attend a Nato summit with impeachment looming. In June 1974, Richard Nixon faced criticism when he headed to Brussels as Nato commemorated its 25th anniversary. He met with fellow alliance leaders and tried to convey a message that he remained fully in power even as the Watergate investigation gained steam. By summer's end, Nixon had resigned.

In contrast, Trump heads to the Nato gathering confident that there aren't the votes needed in the Republican-controlled Senate to convict him should the House vote to impeach him. Even so, he still might not be able to resist throwing attention back to the impeachment inquiry in Washington.

While Nixon remained determined to rise above the swirl of impeachment and pretend he wasn't distracted, Trump can't help himself, said Derek Chollet, executive vice president for security and defence policy at the German Marshall Fund of the United States.

Moreover, for Trump, being in charge hardly means projecting steady confidence. It means proving that he can keep everyone off-balance and nervous about what's coming next.