US president Donald Trump's first conversation with Russian leader Vladimir Putin since taking office is sparking concern among European allies and fellow Republicans about the future of American sanctions on Moscow.

Ahead of the call, Mr Trump has been non-committal about whether he was considering lifting the economic penalties, telling reporters on Friday: "We'll see what happens."

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The European Union also levied sanctions on Russia following its provocations in Ukraine, while Prime Minister Theresa May said after a meeting with Mr Trump that the sanctions should continue.

Two US senate Republicans, John McCain and Rob Portman, are also warning the White House about easing any punishments on Moscow, vowing to turn the sanctions into law if necessary.