US President Barack Obama on Monday called for a full, unimpeded and transparent investigation into the crash of Malaysian airliner MH17 in Ukraine.

"Our immediate focus is on recovering those who were lost, investigating exactly what happened, and putting forward the facts. We have to make sure that the truth is out and that accountability exists," he said in a statement on the crashed Malaysia Airliner in eastern Ukraine, which killed all 298 people on board.

"Now, international investigators are on the ground. They are organized to conduct what should be the kinds of protocols and scouring and collecting of evidence that should follow any international incident like this," Obama said, expressing again concern and condolences over the tragedy.

What they need right now is immediate and full access to the crash site, he said, adding that the investigators need to be able to conduct a prompt and full and unimpeded, as well as transparent investigation.

"Now, Ukrainian President Poroshenko has declared a demilitarized zone around the crash site. But unfortunately, the Russian-backed separatists who control the area continue to block the investigation," he noted.

They have repeatedly prevented international investigators from gaining full access to the wreckage, he said.

"As investigators approached, they fired their weapons into the air. The separatists are removing evidence from the crash site," Obama said, questioning "what are they trying to hide?"

Moreover, he said, the separatists are removing bodies from the crash site, which is an insult to those who've lost loved ones. And Russia has armed them with military equipment and weapons, including anti-aircraft weapons. "Key separatist leaders are Russian citizens," he noted.

"So, given its direct influence over the separatists, Russia and President Putin in particular has direct responsibility to compel them to cooperate with the investigation," Obama said.

The burden now is on Russia to insist that the separatists stop tampering with the evidence, grant investigators who are already on the ground immediate, full and unimpeded access to the crash site, he said.

"President Putin says that he supports a full and fair investigation, but they have to be supported by actions," he said.

Obama reiterated his stand of finding a diplomatic resolution within Ukraine.

The separatists are now risks, not simply to the people inside of Ukraine but the broader international community, and "it's the time for President Putin and Russia to pivot away from the strategy that they've been taking and get serious about trying to resolve hostilities within Ukraine in a way that respects Ukraine's sovereignty."

"Families deserve to be able to lay their loved ones to rest with dignity. The world deserves to know exactly what happened, and the people of Ukraine deserve to determine their own future," the president added.

The West has claimed that pro-independence insurgents in Ukraine's east shot down the airliner by a surface-to-air missile, while Russia challenged the allegations, demanding the United States make public its relevant satellite pictures.