"There is no doubt that this is not a homegrown indigenous uprising,” Obama said from the White House briefing room.
When asked about the prospect of additional sanctions against Russia, the president replied, “I think there are ways for us to deepen or expand the scope of some of that work.”
Senior Ukrainian and NATO officials earlier Thursday said at least 1,000 Russian troops had crossed into southeastern Ukraine, paying little attention to U.S. sanctions already in place against the Kremlin.
"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."
Obama also announced that Ukrainian President Petro Poroshenko would visit the White House next month.
Republicans, however, had called for Obama to be more forceful in his criticism of Russian actions — Obama declined to call the latest Russian military incursion an invasion.
"If reports prove accurate that Putin has in fact sent over 1,000 troops into Ukraine to support and fight alongside Russian-backed separatists, this is an act of war against the sovereign state of Ukraine," said Rep. Buck McKeon, R-Calif., and Rep. Michael Turner, R-Ohio. "This alleged invasion follows Putin’s aggressive armament of Russian-backed separatists, located in Eastern Ukraine's disputed territory, that resulted in the downing of Malaysia Airlines Flight 17 and killed hundreds of civilians last month."
This article was first posted at 5:02 p.m. and has since been updated.