U.S. Secretary of State John Kerry said on Thursday Russian aggression in the east was the greatest threat to Ukraine, and that Washington, while seeking a diplomatic solution, would not "close our eyes" to Russian tanks and fighters crossing the border.

Speaking after talks in Kiev with Ukrainian President Petro Poroshenko, Kerry demanded an immediate commitment by Moscow to a ceasefire in eastern Ukraine that was backed up by actions on the ground.

Russia has consistently rejected accusations from Kiev and the West that it supports a pro-Russian rebellion in eastern Ukraine with troops and heavy weaponry.