President Donald Trump plans to open a dialogue with Russia on Saturday that could lead to lifting US sanctions, even as Britain's visiting prime minister and leading senators from his own party urged him not to let up the pressure on the Kremlin until it reverses its armed intervention in Ukraine.

In what will be their first conversation since Trump took office, he and President Vladimir Putin of Russia intended to talk by telephone on Saturday about areas of possible cooperation, particularly in fighting terrorism in the Middle East, a collaboration which would represent a significant turnabout from years of friction between the two countries. Trump is expected to also speak with Prime Minister Malcolm Turnbull later in the day.

At the direction of the White House, US officials in recent days have been preparing memos outlining possible common ground, including the prospect of removing some or all of the sanctions imposed by former President Barack Obama, according to officials briefed on the matter. Trump has suggested lifting the punitive measures in exchange for nuclear arms cuts and Russian cooperation in fighting the Islamic State.

Asked about sanctions on Friday, Trump played down the possibility of quick action but did not rule it out.

"As for sanctions, very early to be talking about that," Trump said Friday at a White House news conference with Prime Minister Theresa May of Britain. "But we look to have a great relationship with all countries, ideally."

As for Putin, he offered a far more distant assessment after months of praising the Russian president for his leadership. "I don't know the gentleman," Trump said. "I hope we have a fantastic relationship. That's possible, and it's also possible that we won't. We will see what happens."

Standing at Trump's side, May warned against easing sanctions unless Russia abides by a peace settlement for Ukraine negotiated in Minsk, the capital of Belarus. "We believe the sanctions should continue until we see that Minsk Agreement fully implemented, and we've been continuing to argue that inside the European Union," she said.

Any talk of lifting sanctions is all but certain to spark the first serious conflagration between Trump and congressional Republicans, who have largely given the president a pass on a myriad of policy areas where they disagree. Republican lawmakers have been bracing for Trump to make this move, and their concerns deepened Friday when his counsellor, Kellyanne Conway, said in an interview removing sanctions was under consideration.

Senator John McCain warned Trump against lifting sanctions and vowed to push legislation reinstating them if he does, a measure that already has strong bipartisan support, including from Republicans like Senators Rob Portman of Ohio and Ben Sasse of Nebraska. Senate majority leader Mitch McConnell, who has largely shunned confrontation with Trump, has been a long-standing opponent of lifting sanctions, a position he forcefully reiterated Friday.

In a scathing statement against Putin, McCain catalogued all of Russia's controversial actions in Ukraine, Syria and elsewhere and said it could not be trusted as a partner.

"President Trump should remember this when he speaks to Vladimir Putin," McCain said. "He should remember that the man on the other end of the line is a murderer and a thug who seeks to undermine American national security interests at every turn. For our commander in chief to think otherwise would be naive and dangerous."

Portman concurred, saying in a statement, "We must stand by our allies in the region, including Ukraine."

Trump's meeting with May was his first with a visiting foreign leader since taking office with a promise to pursue an "America First" foreign policy. For Trump, it was a debut on the world stage that took on additional meaning after a scheduled White House visit by Mexico's president next week fell apart in a dispute over the border wall Trump wants to build.

Trump appeared comfortable and confident with May standing to his right. He offered a brief opening statement referring twice to the "special relationship" between the two countries, a phrase Britons take seriously. He offered crisp answers, in contrast to Obama, who tended to talk at length. While Trump did not demonstrate detailed policy knowledge, he went out of his way to emphasise commonalities with May.

He also tried to reassure Europeans who view him with deep skepticism. When a British reporter referred to him as a "brash TV extrovert," Trump replied, "Actually, I'm not as brash as you might think."

May, eager to forge a relationship with him akin to Margaret Thatcher's alliance with Ronald Reagan, reciprocated the warm sentiments, praising his "stunning election victory" and conveying an invitation from Queen Elizabeth II for the president to make a state visit, which he accepted.

Addressing one area of disagreement, May said the president had privately expressed his support for NATO, despite past comments disparaging the alliance as "obsolete."

"Mr. President," she said, "I think you said, you confirmed that you're 100 per cent behind NATO."

Trump embraced the decision by British voters to exit the European Union, a referendum known as Brexit he and others have seen as a precursor to his own election. "I think Brexit's going to be a wonderful thing for your country," he said. "I think when it irons out, you're going to have your identity and you're going to have the people that you want in your country."

Trump and May talked about negotiating a new free-trade agreement between the two nations.