U.S. President Donald Trump again expressed frustration on Wednesday with Russian President Vladimir Putin over the intensifying Ukraine conflict, a day after warning that Putin was "playing with fire" by resisting ceasefire talks while escalating drone and missile attacks.

But Trump also told reporters in the Oval Office that he was not yet prepared to impose new sanctions on Russia because he did not want the penalties to scuttle a potential peace deal.

Russia has proposed holding the next round of direct talks with Ukraine on June 2 in Istanbul, Russian Foreign Minister Sergey Lavrov said on Wednesday. There was no immediate response from Kyiv.