Russia has deployed additional troops and military equipment along its border with eastern Ukraine in the past few days, the US and European security agencies have claimed.
More than 30,000 military personnel are believed to have been deployed by Moscow along the Ukrainian border, which represents a significant increase from the figure of 20,000 troops, which was reported by the US and European media last week.
The Russian force includes a large number of motorised units, armoured units, some air support, as well as Special Forces units comprising personnel wearing uniforms with no badges, similar to those worn by soldiers during Russia's recent annexation of the Crimean peninsula, reported Reuters.
Furthermore, Special Forces, airborne troops, air transport and other units that may be required are believed to be mobilising in other locations across Russia.
An unnamed US administration official was quoted by CNN as saying: "This has shifted our thinking that the likelihood of a further Russian incursion is more probable than it was previously thought to be."
When questioned about the 30,000 Russian troops estimate, US President Barack Obama said that the Russian soldiers currently deployed along the Ukrainian border are on Russian soil.
"And if they stay on Russian soil, we oppose what appears to be an effort of intimidation, but Russia has a right, legally, to have its troops on its own soil," Obama said.
"I don't think it's a done deal. And I think that Russia's still making a series of calculations."
Russia, which has its flags raised at all 193 military bases in Crimea, has already denied having plans to invade into the Ukraine.
Meanwhile, the Ukrainian soldiers are currently departing Crimea after the government ordered the withdrawal of all of its troops from the Black Sea peninsula earlier this week.