The US is openly accusing Russia of sending combat forces into Ukraine.
Washington has threatened to tighten economic sanctions, but President Barack Obama has once again ruled out any American military action.
“The violence is encouraged by Russia. The separatists are trained by Russia. They are armed by Russia. They are funded by Russia,” Obama told reporters.
“Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine and the new images of Russian forces inside Ukraine make that plain for the world to see.”
But Washington stopped short of calling Moscow’s intensified support for separatist forces an invasion.
Obama added that he intends to show America’s commitment to Ukraine when he welcomes the country’s President, Petro Poroshenko, to the White House next month.