A Russian convoy of 280 trucks carrying humanitarian aid for Ukraine set off Tuesday amid Western warnings against using help as a pretext for an invasion.

With Ukraine reporting Russia has massed 45,000 troops on its border, NATO said there was a "high probability" that Moscow could intervene militarily in the country's east, where Kiev's forces are closing in on pro-Russian separatists.

Western countries believe that Putin — who has whipped up the passions of Russians with a nationalist campaign in state-controlled media since annexing Crimea from Ukraine in March — could now send his forces into the east to head off a humiliating rebel defeat.