"We assess Malaysia Airlines Flight 17 … was likely downed by a surface-to-air missile, an SA-11, operated from a separatist-held location in Eastern Ukraine," Power said at an emergency meeting of the U.N. Security Council.
Power added that investigators "cannot rule out technical assistance from Russian personnel" since it was unlikely pro-Russian separatists in Ukraine could have operated the SA-11 missile system on their own.
She concluded with an appeal to Russia to de-escalate tensions between Ukraine and the separatists.
"This appalling attack occurred in the context of a crisis that has been fueled by Russian support for separatists and by the Russian failure to follow through on its commitments," Power said.
"Russia can end this war," she added. "Russia must end this war."