British Prime Minister Theresa May urged the United States on Thursday to engage with but "beware" of Russia, as she prepared to meet US President Donald Trump.

"When it comes to Russia, as so often it is wise to turn to the example of President (Ronald) Reagan who -- during negotiations with his opposite number Mikhail Gorbachev -- used to abide by the adage 'trust but verify.' With President Putin, my advice is to 'engage but beware,'" she told Republican lawmakers at a party retreat in Philadelphia.