U.S. President Donald Trump has paused military aid to Ukraine following his clash with Ukrainian President Volodymyr Zelenskiy last week, a White House official said, deepening the fissure that has opened between the two one-time allies. The move comes after Trump upended U.S. policy on Ukraine and Russia upon taking office in January, adopting a more conciliatory stance towards Moscow - and after an explosive confrontation with Zelenskiy at the White House on Friday in which Trump criticized him for being insufficiently grateful for Washington's backing in the war with Russia.