Baltic States – CIS, EU – Baltic States, Legislation, Russia, Ukraine, USA

Two administration officials described the assessment but declined to be identified due to the sensitive nature of the information. The officials emphasized that nothing is certain, but there have been several worrying signs in the past three to four days.

"This has shifted our thinking that the likelihood of a further Russian incursion is more probable than it was previously thought to be," one official said.

The buildup is seen to be reminiscent of Moscow's military moves before it went into Chechnya and Georgia in both numbers of units and their capabilities, the official told CNN.

U.S. military and intelligence officials have briefed Congress on the assessment. As a result, Republican members of the House Armed Services Committee late Wednesday sent a classified letter to the White House expressing concern about unfolding developments.

An unclassified version obtained by CNN said committee members feel "urgency and alarm, based on new information in the committee’s possession."

The committee said there was "deep apprehension that Moscow may invade eastern and southern Ukraine, pressing west to Transdniestria and also seek land grabs in the Baltics."

Committee members noted that General Philip Breedlove, head of the U.S. European Command and NATO military chief, noted the Russians had sufficient forces to make moves into those areas.

American officials believe the more than 30,000 Russian forces on the border with Ukraine, combined with additional Russian forces placed on alert and mobilized to move, give Russian President Vladimir Putin the ability to rapidly move into Ukraine without the United States being able to predict it when it happens.

The assessment indicates that troops on Russia's border with eastern Ukraine are "significantly more" than what is needed for the "exercises" Russia says it has been conducting, and there is no sign the forces are making any move to return to their home bases.

The troops on the border with Ukraine include large numbers of "motorized" units that can quickly move. Additional special forces, airborne troops, air transport and other units that would be needed appear to be at a higher state of mobilization in other locations in Russia.

There is additional intelligence that even more Russian forces are "reinforcing" the border region, according to both officials. All of the troops are positioned for potential military action.

The United States believes that Russia might decide to go into eastern Ukraine to establish a land bridge into Crimea.

The belief is that Russian forces would move toward three Ukrainian cities: Kharkiv, Luhansk and Donetsk in order to establish land access into Crimea. Russian forces are currently positioned in and around Rostov, Kursk, and Belgorod, according to U.S. intelligence information.