A piece of wreckage from the Malaysia Airlines flight MH17 is pictured on July 18, 2014 in Shaktarsk, Ukraine.

A piece of wreckage from the Malaysia Airlines flight MH17 is pictured on July 18, 2014 in Shaktarsk, Ukraine. Dominique Faget—AFP/Getty Images

At least one American citizen was on board Malaysia Airways Flight 17 when it plummeted to earth Thursday, President Barack Obama said Friday, adding that the U.S. government believes that the plane was shot down by a surface-to-air missile fired from separatist-held territory in eastern Ukraine.

Quinn Lucas Schansman, a dual Dutch-U.S. citizen, is believed to be the lone American killed in what Obama called "an outrage of unspeakable proportions."

U.S. Ambassador to the United Nations Samantha Power said Friday at a meeting of the United Nations Security Council that the U.S. government believes the plane was shot down Thursday "by an SA-11 operated from a separatist-held location in eastern Ukraine."

"Because of the technical complexity of the SA-11, it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel," Power said. "Thus we cannot rule out technical assistance from Russian personnel in operating the systems."

Obama called on Russia, Ukraine, and separatist groups to immediately declare a ceasefire so that an independent investigation can begin, adding that any evidence removed from the site must be turned over to investigators.

"If indeed Russian-backed separatists were behind this attack on a civilian airliner, they and their backers would have good reason to cover up evidence of their crime," Power said. "Thus, it is extremely important that an investigation be commenced immediately."

Describing the downing as a "global tragedy," Obama said it was too early to know the intentions of those who shot down the plane. "The eyes of the world are on eastern Ukraine and we are going to make sure that the truth is out," Obama said.

A day after Russian President Vladimir Putin blamed Ukraine for the plane's downing, Power said Russia is responsible for continuing to back Ukrainian separatists despite repeated assurances that they were working to de-escalate the situation.

"Russia says that it seeks peace in Ukraine, but we have continually provided evidence to this council of Russia’s continued support for the separatists," Power said. "Time after time President Putin has committed to working towards dialogue and peace. Every single time he has broken that commitment."

"This war can be ended," Power concluded. "Russia can end this war. Russia must end this war."

Obama said it is time for Putin to put aside propaganda. "He and the Russian government have to make a strategic decision," Obama said, saying “It is not possible for these separatists to function the way they are functioning," including shooting down planes, without Russia's backing.

Russian Ambassador to the U.N. Vitaly Churkin said his government blames the Ukrainian government for the shoot-down, questioning why the plane was even there. “Why did the Ukraine aviation dispatchers send this plane over a war zone," he said, warning against "insinuations" about the attack.