WASHINGTON -- President Obama accused Russian-backed separatists in Ukraine of obstructing the international investigation into the missile attack against a Malaysian Airlines flight and demanded, "Stop tampering with evidence."

Separatists have fired warning shots as investigators have approached the crash site, removed evidence and even made off with the remains of some of the 298 victims, Obama said.

"Which begs the question, what exactly are they trying to hide?" Obama said from the South Lawn of the White House Monday, four days after the disaster.

U.S. officials say they believe a surface-to-air missile battery controlled by separatists shot down the passenger jet over Ukrainian territory they control.

The president called on Russia put pressure on the separatists to adhere to a cease fire and let the investigation proceed.

"Given its direct influence over the separatists, Russia — and President Putin in particular — has a direct responsibility to compel them to cooperate with the investigation. That's the least they can do," Obama said.

"Time is of the essence. Our friends and allies need to be able to recover those who were lost. That's the least we can do," Obama said. "The world deserves to know exactly what happened, and the people of Ukraine deserve to determine their own future."

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