The probability of Russia invading eastern Ukraine is higher than ever, according to US intelligence assessments.

"The thinking in the US government is that the likelihood of a major Russian incursion into Ukraine has increased," a senior US official said.

US officials are reacting with alarm to Russia's buildup of military forces along the Russia-Ukraine border, Russian President Vladimir Putin's rhetoric and his claims that ethnic Russians face "brutality" in Ukraine.

US defence officials believe that the amount of troops stationed on the Ukraine border is far more than a training exercise demands. US observers are concerned with the fact that none of the Russian troops have returned to their bases.

Early estimates suggested that 30,000 troops lined the border but Republican Michael Turner has suggested that as many as 80,000 could have been mobilised with a recent surge of 50,000 troops alleged.

"I can't tell you how awful this is," one congressional source told Fox News.

If Russian troops were to incur upon the Ukrainian mainland, it would be "far from a bloodless event as we saw in Crimea" as Ukraine would attempt to defend itself, said an unnamed defence official.

Russian troops have seized three military bases in Crimea following President Vladimir Putin's annexation of the Russian-dominated region.

There are now concerns that Putin has set his sights on another area with a prominent Russian-speaking population, Transnistria.

Gen. Philip Breedlove, Nato's supreme allied commander for Europe has expressed deep concern with the Russian military's moves.

"We're all concerned about what Russia is doing on the border of Ukraine," Breedlove said after the first briefing. "The size of the forces have a message that are not congruous with respecting the borders."

Republicans have written a letter to US President Barack Obama demanding he take stronger action against the Putin regime as they are "gravely concerned" about the troop buildup.

"There is deep apprehension that Moscow may invade eastern and southern Ukraine, pressing west to Transdniestria, and also seek land grabs in the Baltics," they wrote.