NATO warned on Wednesday that Russia's military presence on the flashpoint border with Ukraine was of "grave concern" and that Moscow's forces could reach military objectives in just days.
General Philip Breedlove, NATO's top commander, told the Wall Street Journal that Russian troops could invade vast portions of Ukrainian territory in three days to five days.
NATO Secretary General Anders Fogh Rasmussen said he shared that assessment and warned of a "a very massive Russian military buildup along the Ukrainian border" which NATO estimates at about 40,000 troops.
"This is really a matter of grave concern," Rasmussen said, at the close of a NATO foreign ministers meeting in Brussels.
Breedlove said potential objectives for Russia included a land corridor linking Crimea and mainland Russia, the taking of the strategic Odessa port or an occupation of the breakaway Russian-speaking region of Transdniestr in Moldova.
Rasmussen warned that if Russia were to intervene further in Ukraine, he "would not hesitate to call it an historic mistake."
"It would be a miscalculation with huge strategic implications," he said.
The warning came the day after NATO announced the alliance was suspending all practical cooperation with Russia, military and civilian, and that there was no confirmation that Russian troops were pulling back from the border.