Speaking at the White House, the president accused the pro-Russian rebels of tempering with evidence and hampering the probe into the crash.
Malaysian Airlines Flight MH17 went down in Ukraine four days ago, killing all 298 people on board.
"Our immediate focus is on recovering those who were lost, investigating what exactly happened and putting forward the facts. We have to make sure that the truth is out," President Obama said.
Ukraine's government and the separatists have accused each other of shooting down the aircraft.
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