Nato's top military chief Philip Breedlove has warned that Russia was ready to invade Ukraine and could "accomplish its objectives" in three to five days.
Breedlove is to submit a "reassurance package" to Nato respresentatives as the military alliance responds to Russia's "incredibly concerning" mobilisation of troops along its border with Ukraine.
Breedlove has produced a list of options for Nato representatives that cover land, sea and air and incorporate northern, central and southern areas of Russia's western border.
"These actions by Russia clearly should cause us to rethink our positioning and posturing of Nato forces," said the commander.
The 40,000 Russian troops stationed on the eastern Ukraine border were "a very large and very capable and very ready force", he added.
Foreign ministers have pledged beef up Ukraine's security in the face of what they described as Russia's challenge to freedom and peace in Europe.
The 28-member alliance said it would suspend all civilian and military cooperation with Moscow over Russia's "illegal military intervention" in Ukraine.
"We do not recognise Russia's illegal and illegitimate attempt to annex Crimea," they said after a closed-door meeting at Nato headquarters in Brussels.