REUTERS - President Barack Obama said on Friday that a surface-to-air missile fired from territory controlled by Russian-backed separatists in Ukraine was responsible for shooting down a Malaysian passenger plane on Thursday.

Obama said Russian President Vladimir Putin had the most power to reduce the violence in Ukraine and had not chosen to do so. He said Ukraine separatists had received a steady flow of support from Russia, including anti-aircraft weapons.

Obama's comments were the strongest public assertions yet from the United States that pro-Russian separatists likely brought down the plane and that Moscow bears some responsibility by allowing sophisticated Russian arms to flow into eastern Ukraine.

If Putin made the decision that to stop the flow of armaments and fighters into Ukraine, it would stop, Obama said.

Obama called the incident an "outrage" of unspeakable proportions. So far, officials had determined that only one U.S. citizen had been on board the plane.

"This was a global tragedy," Obama said, calling for a credible international investigation into what happened.

He said Russia, pro-Russia separatists and Ukraine must adhere to an immediate ceasefire.

"It's important for us to recognize that this outrageous event underscores that it is time for peace and security to be restored in Ukraine," he said.

Most of Obama's remarks were focused on Russia. Time and again, he said, Moscow had refused to take the necessary steps to de-escalate the situation in Ukraine.

"If Mr. Putin makes a decision that we are not going to allow heavy armaments and the flow of fighters into Ukraine ... then it will stop," Obama said. "And if it stops, then the separatists will still have the capacity to enter into negotiations and try to arrive at the sort of political accommodations that Mr. Putin himself says he wants to see."