Russia has all the forces it needs on the border to invade Ukraine within the next three to five days, Nato’s top military commander has warned.
The Supreme Allied Commander in Europe and the US Air Force General Philip Breedlove described the situation at the border as “ incredibly concerning”.
He said Nato had spotted signs of movement by a very small part of the Russian force overnight but there was no indication it was returning to barracks.
"This is a very large, very capable and very ready force" Mr Breedlove told 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," Mr Breedlove warned.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
3/12 Ukraine crisis
Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
4/12 Ukraine crisis
Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
7/12 Ukraine crisis
An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
10/12 Ukraine crisis
An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
11/12 Ukraine crisis
Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
12/12 Ukraine crisis
Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
"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 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.
"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," he added.
On Wednesday, Russia accused Nato of reverting to Cold War language by suspending all practical cooperation with Moscow the day before in protest at its annexation of Crimea from Ukraine.
Nato also ordered military planners to draft measures to strengthen its defences and reassure nervous eastern European countries.
Mr Breedlove said the ministers had asked him to draw up by a package of measures that would include reinforcements by land, air and sea by 15 April.
Additional reporting by agencies