Russia has massed all the forces it needs on Ukraine's border if it were to decide to carry out an "incursion" into the country, and it could achieve its objective in three to five days, NATO's top military commander said on Wednesday.
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 that this was part of a withdrawal 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 the United States and Europe to impose sanctions on Moscow. They have said they will strengthen these if Russia moves beyond Crimea into eastern Ukraine.
NATO military chiefs are concerned that the Russian force on the Ukrainian border, which they estimate stands at 40,000 soldiers, could pose a threat to eastern and southern Ukraine.
"This is a very large and very capable and very ready force," Breedlove said in an interview with Reuters and The Wall Street Journal.
The Russian force has aircraft and helicopter support as well as field hospitals and electronic warfare capabilities - "the entire suite that would be required to successfully have an incursion into Ukraine, should the decision be made," Breedlove said.
"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."
Russia has said it has no intention of invading its neighbour, although since the toppling of Moscow-backed Ukrainian President Viktor Yanukovich in February it has asserted a right to intervene to protect ethnic Russians if necessary.
Further Russian intervention in Ukraine would be an "historic mistake" that would deepen Moscow's international isolation, NATO Secretary-General Anders Fogh Rasmussen said on Wednesday.
Breedlove said Russia could have several potential objectives. These included: 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 the east of the former Soviet republic if Moscow ordered them to do so, Breedlove said.
He said NATO planned no military response to events in Ukraine, which is not a member of the alliance: "We really need to resolve this in a peaceful manner because a military conflict would be very costly to Europe and to the nations involved."
But events in Ukraine, particularly Russia's use of a snap exercise to ready a military force to intervene in Crimea, are already leading to a deep rethink of strategy by NATO, a military alliance of 28 nations that has been the core of European defence for more than 60 years.
"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, etc," Breedlove said.
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 of a threat from Russia.
Breedlove said the ministers had asked him to draft by April 15 a package of measures that would include reinforcements by land, air and sea. He said he would offer a range of options and recommend a plan.
"We will work on air, land and sea 'reassurances' and we will look to position those 'reassurances' across the breadth of our exposure: north, centre, and south," he said.
The United States deployed a warship to the Black Sea last month for exercises with allies. It also increased the number of US aircraft in regular NATO air patrols over the Baltics and beefed up a previously planned training exercise with the Polish air force.
A number of other NATO allies have also offered extra planes and ships to reinforce eastern European countries.
Breedlove said the initial US moves with "aircraft in the north, aircraft in the centre and a ship in the south" could serve as a model for what NATO would do next, except other NATO allies would also be involved in future reinforcements.
He would not say how long the reinforcements would stay in central and eastern Europe but said it would not be short-term, although deployments would be periodically reviewed.
NATO may bring forward some military exercises or expand others, he said.
NATO is also looking at the readiness of its rapid reaction force, the NATO Response Force, which includes land, air, sea and special operations forces, and could consider shortening the time it takes for the force to respond, he said.
Plans for NATO and Russia to cooperate in providing a naval escort for a US ship that will destroy Syria's deadliest chemical weapons were scrapped due to the Crimea crisis.
But Breedlove said NATO members had made plenty of offers of ships to escort the US cargo ship Cape Ray and he hoped this task would become an alliance mission.
The Ukraine crisis has reopened debate within NATO about whether its members need to spend more on defence.
Many of them have cut military budgets in response to the financial crisis, with only a handful reaching NATO's target of spending 2 per cent of economic output on defence.
Breedlove said countries needed to get to 2 per cent, but this was not a 'magic' figure; the point was to spend it "to create the kind of requirements and capabilities that we need as a NATO team".