The Pentagon says Russian combat troops are again building up along the border with Ukraine, and officials believe that Moscow may be sending heavy weapons into the country to aid the separatists.
Army Col Steve Warren says the US believes there are now up to 12,000 Russia troops on the border, reflecting a steady increase in recent weeks.
US officials in May estimated that there were as many as 40,000 Russian troops arrayed along the border but that number had recently fallen to about 1,000 under pressure from the West.
Warren says there are a variety of Russian forces there, including combat battalions.
Ukraine and the West have accused Russia of fomenting the insurgency by sending troops and weapons, including tanks and rocket launchers, something Moscow denies.
