Over 10,000 Russian soldiers in combined arms battalion tactical units are massed at the border, Xinhua quoted Pentagon Press Secretary John Kirby as saying in a press conference.
These battalion groups consist of infantry, armour and artillery, and also have organic air defence capabilities, the admiral said. "They are capable across a wide spectrum of military operations."
Kirby called these forces capable and ready, and said they were closer to the Ukrainian border than Russian forces were in the spring. "We continue to see advanced weapons systems moving across the border and being provided to the separatists."
On Monday, US President Barack Obama spoke on Ukraine situation in a video conference call with British Prime Minister David Cameron, French President Francois Hollande, German Chancellor Angela Merkel and Italian Prime Minister Matteo Renzi.
The leaders agreed on the importance of coordinated sanctions measures on Russia for its continued transfer of arms, equipment, and fighters into eastern Ukraine.