US President Donald Trump said he wanted to have good relations with Russia and declined to say whether he was ready to lift sanctions on Moscow, which visiting British Prime Minister Theresa May said must stay in place.

Speaking at a news conference after his first meeting with a foreign leader at the White House since becoming president, Trump said he hoped to have a "fantastic relationship" with Russian President Vladimir Putin, but he said it was possible that would not occur.

Any move by the White House to lift sanctions against Russia for its actions in Ukraine would likely cause consternation among European allies who believe sanctions should be lifted only if Moscow complies with the West's conditions on Ukraine.

Those concerns would be compounded at home, where many in the US Congress are vehemently opposed to Putin's actions in Ukraine and Syria as well as what US intelligence agencies have concluded was Russian meddling in the US election campaign.

May told the news conference she believed sanctions against Russia should stay in place until Moscow fulfilled its obligations under the Minsk agreement.

She also made a point of emphasizing that during their talks, Trump had given strong backing to NATO, an alliance that the president has previously called obsolete.

Putin and Trump are likely to discuss the sanctions when the two leaders speak by telephone tomorrow, a senior White House aide said.

Trump said he would defer to Defense Secretary James Mattis regarding the use of waterboarding as an intelligence-gathering tool, even though Trump said he still believes the practice works.

Trump said Mattis does not "necessarily believe" in waterboarding and other enhanced interrogation techniques, which many lawmakers and other rights groups denounce as torture.

"I don't necessarily agree, but I would tell you that he will override because I'm giving him that power," Trump said of Mattis.

"I happen to feel that it does work. I've been open about that for a long period of time. But I am going with our leaders," Trump said.

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