Earlier, Breedlove said an invasion could begin in the next three to five days.
Western intelligence agencies have expressed concern at what they have described as worrying troop build-ups on Ukraine's border in the past week. While journalists in the border region have failed to find signs of large troop movements, security sources have said they believe the troops are camouflaged or located on Ministry of Defence land closed to the public.
Sergei Lavrov, Russia's Foreign Minister, has said Russia has no interest in moving troops across the border.
Breedlove's comments come after Nato suspended all military and civilian co-operation with Russia in response to the annexation of Crimea last month. Nato commanders have also been ordered to devise ways to better protect alliance members that feel threatened by Russia.
At a meeting of alliance foreign ministers on Wednesday, Poland asked Nato to station 10,000 troops on its territory.