Speaking from the White House briefing room, Obama stressed that an investigation into the plane crash is ongoing, and that "we don't yet know exactly what happened."
Obama cautioned against misinformation and outlined what facts are known so far, including Russia's role in the unrest.
"Evidence indicates the plane was shot down by a surface-to-air missile launched from an area that is controlled by the Russian-backed separatists inside Ukraine.
"We also know that this is not the first time a plane has been shot down in eastern Ukraine. Over the last several weeks, Russian-backed separatists have shot down a Ukrainian transport plane and a Ukrainian helicopter and they claimed responsibility for shooting down a Ukrainian fighter jet.
"Moreover, we know that these separatists have received a steady flow of support from Russia. This includes arms and training. It includes heavy weapons. And it includes anti-aircraft weapons."
Launching into "what must happen now," the president noted that FBI and National Transportation Safety Board personnel "are on their way" to aid the international investigation, which was endorsed Friday by the U.N. Security Council.
Obama called for Russia, pro-Russian separatists, and Ukraine to "adhere to an immediate ceasefire" to aid the investigation. He also cautioned against evidence tampering and stated that "Investigators need to access the crash site."
MH17 crashed in rebel-controlled Donetsk, complicating the task of securing the crash site and causing confusion over who had taken possession of the plane's audio and voice recording devices.
The president emphasized that Russian President Vladimir Putin has a significant role to play in restoring "peace and security" to neighboring Ukraine.
"... this outrageous event underscores that it is time for peace and security to be restored in Ukraine.
"... time and again, Russia has refused to take the concrete steps necessary to de-escalate the situation.
"... Instead, it has continued to violate Ukrainian sovereignty and to support violent separatists. It has also failed to use its influence to press the separatists to abide by the ceasefire.
"... Now is the somber and appropriate time to take a hard look at what has happened. Violence and conflict inevitably lead to unforeseen consequences.
"Russia, the separatists, and Ukraine all have the capacity to put an end to the fighting."
In talks with Putin, Obama said he has made it clear that "he and the Russian government have to make a strategic decision. Are they going to continue to support violent separatists whose intent is to undermine the government of Ukraine, or are they going to work with the government of Ukraine to arrive at a ceasefire and a peace that takes into account the interests of all Ukrainians."
When pressed for more details on the party responsible for shooting down Flight 17, Obama acknowledged that "we're beginning to draw some conclusions," and those conclusions point toward Russian-provided equipment and training to separatists in eastern Ukraine.
"A group of separatists can't shoot down military transport planes or, they claim, shoot down fighter jets, without sophisticated equipment and sophisticated training. And that is coming from Russia.
"Given the nature of the shot that was fired, there are only certain types of anti-aircraft missiles that can reach up 30,000 feet and shoot down a passenger jet. We have increasing confidence that it came from areas controlled by the separatists but without having a definitive judgment on those issues yet, what we do know is that the violence that's taking place there is facilitated, in part, in large part, because of Russian support. And they have the ability to move those separatists in a different direction."
The president noted that additional information will likely surface in the coming days, weeks, and over the next month.