The United States has begun building a case linking pro-Russian separatists to the shocking downing of the Malaysia Airlines jet in Ukraine.
A command centre has been set up at the US State Department, where officials gathered for a briefing from the CIA on the political and military situation.
A sombre President Barack Obama declared the deaths of those on board, including at least one American, an "outrage of unspeakable proportions".
He said the US believes the Boeing 777 was felled by a surface-to-air missile launched from an area near the Ukraine-Russia border that is controlled by Kremlin-backed separatists.
Even as he warned that the exact circumstances were still being determined, the president turned his sights on Russia, saying the insurgents would not be capable of carrying out such an attack without Moscow's support.
"We know that they are heavily armed and they are trained, and we know that that's not an accident," Mr Obama said. "That is happening because of Russian support."
The president spoke shortly after Samantha Power, the U.S. ambassador to the United Nations, outlined preliminary evidence against Russia and the separatists during an emergency meeting of the UN Security Council.
She said separatists were spotted on Thursday with an SA-11 anti-aircraft missile at a location close to the site where the plane came down and that they had boasted on social media sites about shooting down a plane, then later deleted those posts.
Ms Power joined Mr Obama in calling for an immediate international investigation, warning that the separatists and those supporting them would have "good reason to cover up evidence of their crime".
The US has called for evidence from the crash site to remain in Ukraine until investigators determine who is responsible.
The American killed in the incident was identified as Quinn Schansman. Officials are still working to confirm whether any other US citizens were on board the plane.
For Mr Obama, the downed plane adds new complexity to US efforts to quell the months-long conflict between Russia and Ukraine. Increasingly stringent economic sanctions imposed by the US and Europe, including a new round of penalties announced a day before the plane was shot down, have done little to change Russian president Vladimir Putin's approach.
Mr Obama warned Russia anew yesterday that the US has the capacity to increase the economic pain, but outlined no specific potential actions. He did say he saw no US military role in the conflict that has stemmed in part from Russia's annexation of the Crimean Peninsula from Ukraine.
All 298 people aboard the Malaysian plane were killed in Thursday's incident. The passengers, including scores of children, came from a dozen countries, spreading the impact of the Ukraine crisis around the world.
"This certainly will be a wake-up call for Europe and the world that there are consequences to an escalating conflict in eastern Ukraine - that it is not going to be localised, it is not going to be contained," Mr Obama said.
The president has spoken with world leaders since the crash, while vice president Joe Biden called prime minister Donald Tusk of Poland, a Nato ally alarmed by Russia's actions in neighbouring Ukraine.
Mr Obama learned of the plane crash during a phone call with Mr Putin. Yesterday the president stopped short of blaming Mr Putin for the downing of the plane, but said the Russian leader could bring the broader conflict to an end.
"He has the most control over that situation, and so far at least he has not exercised it," Mr Obama said.
A US official said all available evidence, including satellite imagery, pointed to the plane being shot down with an SA-11 anti-aircraft missile fired by pro-Russian separatist forces.
The US has detected three separate events associated with the shootdown: the launching of the missile from the Ukraine side of the border, the missile's impact with the plane and the plane slamming into the ground.
The State Department said the FBI and National Transportation Security Board are each sending at least one agent to Ukraine, and perhaps more later, to assist with the crash investigation.