U.S. intelligence now estimates the Russian troop buildup near the Ukraine border at 40,000 to 50,000, according to CBS News correspondent David Martin.
The increase is due to the continued arrival of logistical units - for instance, field hospitals - that would be needed to sustain operations. The intent of the buildup is still not known but the Russians have assembled "all the elements of combat power" needed to invade, according to officials.
Analysts have come up with four potential scenarios that could unfold, Martin reports. First, the Russians could cut all the way across Ukraine to the pro-Russian enclave of Transdniester in Moldova; second, they could try to take Kiev; third, they could take the predominantly Russian-speaking cities of southeastern Ukraine; and finally, they could take the cities necessary to establish a land corridor to Crimea.
Russia has the forces to execute any of these scenarios, Martin reports, and they are ready to go with little or no notice. As with all buildups, a military force can only sustain a peak level of readiness for so long before it starts to decay - the old "use it or lose it" syndrome. They could use it by invading, conducting an exercise or standing down.
The Russian buildup is taking place 30 to 120 kilometers back from the border. If the Russians go into Ukraine, Martin expects news of any invasion would come from people on the ground, which is the way it happened in Crimea.
CBS Evening News" anchor and managing editor Scott Pelley, acknowledged Russia's troop buildup and said Moscow must take steps to reduce tensions over Ukraine.
"It's well known and well acknowledged that you've seen a range of troops massing along that border under the guise of military exercises. But these are not what Russia would normally be doing. And, you know, it may simply be an effort to intimidate Ukraine or it may be that they've got additional plans," Mr. Obama said.
To resolve and de-escalate the situation right now, Mr. Obama said Russia needs "to move back those troops and to begin negotiations directly with the Ukrainian government, as well as the international community."
Mr. Obama said Russian President Vladimir Putin has been "willing to show a deeply held grievance about what he considers to be the loss of the Soviet Union."
The U.S. president add that Putin is "misreading the West" with regards to Ukraine.
"He's certainly misreading American foreign policy," Mr. Obama said. "We have no interest in encircling Russia and we have no interest in Ukraine beyond letting the Ukrainian people make their own decisions about their own lives."