United States (US) President Donald Trump is set to meet his first world leader since taking office British Prime Minister Theresa May, a friendly ally who hopes to nudge the populist president toward the political mainstream.

The visit Friday comes a day after Mexican President Enrique Pena Nieto called off his own trip to Washington, planned for next week, amid wrangling over who will pay for Trump's planned wall along the US-Mexico border. Trump's spokesman said the president would seek a 20 per cent tax on Mexican imports to pay for the barrier, then later clarified that such a tax would be a possible approach.

May's meeting with the president in the Oval Office is being hailed by the British government as a sign that the trans-Atlantic "special relationship" is valued by the new administration.

The White House said late Thursday that May and Trump would hold talks, followed by a press conference and a working lunch in a statement that misspelled the prime minister's first name as Teresa. It was later corrected.

May's visit, so soon after Trump's inauguration, has been criticised by her political opponents, and risks being overshadowed by the flood of announcements, plans and proposals coming out of the White House.

On Thursday May was repeatedly asked about Britain's stance on torture the UK condemns it after Trump said he thinks torturing terrorism suspects works.

Trump is something of a mystery to world leaders, many of whom expected Democrat Hillary Clinton to win the election.

His administration's main interlocutors with foreign governments so far including son-in-law Jared Kushner and senior adviser Steve Bannon, a conservative media executive are also unknown to many of their counterparts.

So May is a bit of a scouting party or guinea pig among global politicians.

She has strong reasons for wanting the relationship to work. Britain is set to leave the European Union and its 500 million-person single market. A trade deal with the US, Britain's biggest export market, is a major prize.

Trump has drawn parallels between Britain's choice to leave the EU and his own success, using the Brexit vote to bolster his derision of the 28-nation bloc and his preference for striking bilateral agreements.

That puts May in an awkward spot. She wants a good relationship with Trump, but does not share his disdain for the EU, saying it's in Britain's interests that it succeed.

Trump and May both addressed a Republican retreat Thursday in Philadelphia, though their visits did not overlap.

During his remarks to lawmakers, Trump bemoaned the fact that Wilbur Ross, his nominee to be commerce secretary, would not be confirmed in time for a visit that was expected to focus heavily on trade.

"I don't have my commerce secretary," Trump said. "They want to talk trade. So I'll have to handle it myself." May's speech alternated between saluting Trump's vision for what she called American "renewal" and reminding him, and his Republican colleagues, of the United States' global responsibilities.

May urged the US to join Britain and embrace "the responsibility of leadership in the modern world."

She mentioned the trans-Atlantic alliance during World War II and afterward, when the UK and the US set up the United Nations and Nato organisations Trump has strongly criticized.

She joined in Trump's criticism of previous US governments' foreign policy, saying that "the days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over."

But May also said Britain supported a strong European Union and considered Nato the bulwark of global security.

May praised Trump's dedication to fighting the militant Islamic State group and "radical Islamic terrorism", but stressed the need not to tarnish "the peaceful religion of Islam and the hundreds of millions of its adherents."

May acknowledged the need to engage with Russia to end the war in Syria, yet cautioned that the West's approach to President Vladimir Putin should be "engage but beware."

There are few obvious similarities between the brash businessman Trump and the quiet, diligent May.

In her speech, May referred several times to Republican President Ronald Reagan and Conservative Prime Minister Margaret Thatcher politicians from very different backgrounds who forged an alliance that helped transform their parties, and the world.

Speaking about Trump, May said aboard her plane, "sometimes opposites attract", signalling a shift in foreign policy that will bring her position more in line with the US president.

'Britain becoming US vassal state'

Britain is becoming subservient to a US that will be extremely difficult to cooperate with judging by President Donald Trump's "serious and worrying" first acts, French presidential candidate Emmanuel Macron said on Friday.

"Britain lived in an equilibrium with Europe," Macron told France Culture radio. "But now it is becoming a vassal state, meaning it is becoming the junior partner of the United State."

French opinion polls show Macron, a former investment banker running as an independent in this spring's presidential election, closing in on the two frontrunners, conservative candidate Francois Fillon and far-right leader Marine Le Pen.

Macron said Trump's early policy moves suggested the United States might become a destabilising force on the world stage.

"What's happening today with Trump's first statements and choices is extremely serious and worrying," Macron said.

"It's firstly a choice that it will be an America that provokes... an America that destabilises things that have been built for decades."

Macron has previously said he was sure Trump would maintain close ties with the European Union, but on Friday he appeared to concede this may no longer be the case.

"It signifies that the US will no longer be in a position to co-organise globalisation and be the world's policeman with the European Union," the former economy minister said.

"The unpredictable choices, the outbursts and the inward-looking United States of Trump no longer guarantees Europe's security."