WASHINGTON (AFP/BLOOMBERG) - Britain's Theresa May said US President Donald Trump vowed '100 per cent' support for the Nato alliance as she became the first foreign leader to visit his White House, despite his earlier characterisations as the US-European trade group as 'obsolete'.

The meeting was a pivotal moment in trans-Atlantic relations, which have been rocked by Trump's election and his willingness to rethink NATO, the UN and other foundation blocks of the liberal world order.

Trump greeted May himself upon her arrival at the White House. Her trip is equal parts influence campaign and charm offensive.

Trump said he would have a "fantastic" relationship with May, promising to work closely on trade and defence as her country exits the European Union, but the two leaders differed over continuing sanctions against Russia.

Trump and May held a cordial, 18-minute news conference at the White House after a private meeting.

Trump said that the so-called Brexit, under which Britain would impose immigration curbs and recraft its trade relationships, would be "a wonderful thing" for Britain.

"People want to know who's coming into their country and they want to control their own trade and various things," Trump said. "When it irons out you're going to have your own identity and going to have the people you want in your country and going to be able to make free trade deals without someone watching you."

May and Trump posed for photos in front of a bust of Winston Churchill before reporters were ushered out of the office. Trump was joined for the meeting by his son-in-law and senior adviser Jared Kushner, chief strategist Stephen Bannon and National Security Adviser Michael Flynn.

"We pledge our lasting support to this most special relationship," Trump said at a news conference after their meeting. "A free and independent Britain is a blessing to the world and our relationship has never been stronger."

May congratulated Trump on his "stunning election victory".

However, the two leaders differed over maintaining economic sanctions against Russia to punish the country for its military incursions into Ukraine. May said the sanctions should continue until Russian President Vladimir Putin complied with a cease-fire agreed to in Minsk in 2014. Trump will speak with Putin by phone today (Jan 28, 2017) and said it was "very early" to discuss whether the sanctions should be lifted.

"I don't know the gentleman," he said of Putin. "I hope we have a fantastic relationship. That's possible and it's also possible that we won't."

Trump is beginning to implement his vision for a more assertive US approach to trade relationships, while May is going all-out to maintain the close ties between the two nations. She made the unusual gesture on Thursday of delivering a speech only to Republican members of Congress gathered at a retreat in Philadelphia.

After a British reporter asked Trump to explain his "alarming views" to British viewers, Trump joked to May: "This was your choice of a question. There goes that relationship."

May planned to pitch a free-trade deal to Trump even as a new era of US protectionism dawns. The first target for Trump is Mexico, which he has blamed in part for the loss of US manufacturing jobs. The escalating row over Trump's planned border wall and border tax saw relations between the neighbouring countries sink to a low point virtually overnight.

May told Republicans in Philadelphia that she was in the US "to renew the special relationship for this new age". She also complimented the GOP for its election victories, saying it was an "honour" to visit as "dawn breaks on a new era of American renewal."

May called for a retreat from the interventionist policies favoured by George W Bush and Tony Blair during their overlapping terms a decade ago.

"The days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over," she said. "But nor can we afford to stand idly by when the threat is real and when it is in our own interests to intervene."

She is pursuing an exit from the EU that British voters approved last year, and has been encouraged by Trump's administration. One item on the agenda for both leaders is groundwork for a trade agreement that would take effect once Britain completes Brexit.

They were also discussed the Nato (North Atlantic Treaty Organisation), which Trump has called "obsolete" but that May still sees as an important international institution. Britain provides the second-largest contribution to Nato, after the US.

At the beginning of the news conference, May said that Trump had confirmed his support for NATO. She said she agreed with Trump that members of the alliance should boost their defence spending.

While the two leaders are stylistically different - "opposites attract", May joked to reporters during her flight to the US- they both came to power following votes that were widely seen as a rejection of elites and a shift toward more inward-facing foreign policies.

"The people sent a very clear message, that they wanted to go outside the establishment, they want these two countries to behave as sovereign nations that put their people first, stop the flood of refugees in without knowing who they are or why they are there, boost wages, promote job creation and frankly be independent from a lot of these establishment organisations whether the EU in Europe or the establishment organisations in the US," Conway said Friday.

The relationship between the two leaders got off to a bit of a rocky start. In December 2015, May, who was then Britain's home secretary, criticised Trump's proposal to ban Muslims from entering the US as "divisive, unhelpful and wrong".

After Trump's election, it took more than a day for the two leaders to talk, by which point he'd already spoken to several others, including leaders in Australia and Ireland.

May has been eager to strengthen her team's relationship's with Trump's. She sent her two most senior aides, Nick Timothy and Fiona Hill, on a secret trip to the US in mid-December to meet with members of Trump's transition staff, and also sent other senior British officials to meet with top Trump aides after the election.

May also defended the Iranian nuclear deal against the president's criticism, saying it was "vitally important" for regional security - but must now be properly enforced.