Calling the situation "incredibly concerning", Nato's supreme allied commander in Europe, US Air Force General Philip Breedlove, said Nato had spotted signs of movement by a very small part of the Russian force overnight but had no indication it was returning to barracks.
Russia's seizure and annexation of Ukraine's Crimea region has caused the deepest crisis in East-West relations since the Cold War, leading Europe and America to impose sanctions on Moscow. They have said they will strengthen those sanctions if Russia moves beyond Crimea into east Ukraine.
Nato military chiefs are concerned the Russian force on the Ukrainian border, which they estimate stands at 40,000 soldiers, could pose a threat to eastern and southern Ukraine.
Mr Breedlove said: "This is a very large and very capable and very ready force."
The Russian force has aircraft and helicopter support, as well as field hospitals and electronic warfare capabilities.
The air force general added: "(It's) the entire suite that would be required to successfully have an incursion into Ukraine should the decision be made. We think it is ready to go and we think it could accomplish its objectives in between three and five days if directed to make the actions."
He said Russia could have several potential objectives, including an incursion into southern Ukraine to establish a land corridor to Crimea, pushing beyond Crimea to Ukraine's Black Sea port of Odessa or even threatening to connect to Transdniestria, the mainly Russian-speaking, separatist region of Moldova that lies to the west of Ukraine.
Russia also has forces to the north and northeast of Ukraine that could enter eastern Ukraine if Moscow ordered them to do so, Mr Breedlove said. Any such actions would have far-reaching implications for Nato, the military alliance of 28 nations that has been the core of European defence for more than 60 years.
The general said: "We are going to have to look at how our alliance now is prepared for a different paradigm, a different rule set. We will need to rethink our force posture, our force positioning, our force provisioning, readiness."
Nato foreign ministers meeting in Brussels this week suspended all practical cooperation with Russia in protest at its actions in Crimea and asked military commanders to draw up plans to reinforce Nato members in eastern Europe that are fearful about a threat from Russia.
Mr Breedlove said the ministers had asked him to draw up a package of measures by April 15 that would include reinforcements by land, air and sea.
Meanwhile, Russia has accused Nato of reverting to the language of the Cold War by suspending cooperation with Moscow.
It said the decision by Nato foreign ministers to suspend all practical cooperation with Russia created a sense of "deja vu".
Foreign Ministry spokesman Alexander Lukashevich said: "The language rather resembles the verbal jousting of the Cold War era."