Obama pressures on Russia, Putin in Ukraine crash probe

US President Barack Obama piled pressure on Russian President Vladimir Putin on Monday to force pro-Russian separatists to stop blocking an international investigation into the shootdown of a passenger jet last week.

Obama denounced the Russian role in eastern Ukraine in some of his strongest language yet and pointedly appealed to Putin to cut ties with the separatists or risk greater international isolation.

"Now'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," Obama said in remarks on the White House South Lawn.

With investigators blocked from access to the crash site in eastern Ukraine and most of the bodies of the victims removed, Obama said Russia should compel the separatists to let the investigation take place. He questioned why the rebels are blocking access.

"What are they trying to hide?" he said.

Obama did not specifically threaten new economic sanctions against Russia, but he hinted at costs to come should Moscow not change course.

If Russia continues to back the rebels and they become risks not just to Ukraine but to the broader international community, "the costs for Russia's behaviour will only continue to increase," he said.

US officials are increasingly confident that the Malaysian Airlines plane was shot down by the separatists with a Russian-made surface-to-air missile.

"Russia has trained them. We know that Russia has armed them with military equipment and weapons, including anti-aircraft weapons. Key separatist leaders are Russian citizens," Obama said.

Given Russia influence over the rebels, he said, "Russia, and President Putin in particular, has direct responsibility to compel them to cooperate with the investigation. That is the least that they can do."

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