WASHINGTON (AP) — President Barack Obama sternly called on Russian President Vladimir Putin to compel Kremlin-backed separatists to stop hampering the probe at the Ukraine site of a downed passenger jet and allow international investigators unfettered access.

"We have to make sure the truth is out and accountability exists," Obama said Monday from the South Lawn of the White House.

Obama accused the separatists of removing evidence and bodies from the site, actions that he said raise the question of "what exactly are they trying to hide?"

The White House has been seeking to pin responsibility on the separatists for shooting down the Malaysia Airlines plane carrying nearly 300 people. Officials have also pointed a finger at Russia, arguing that it would be all but impossible for the insurgents to operate the sophisticated missile system needed to shoot down a passenger jet without some level of assistance from Russia.

Obama offered no new evidence Monday as to who was responsible for shooting down the plane.

Behind the scenes, the U.S. was also pressing the European Union to ratchet up economic sanctions on Russia for its actions in Ukraine. The EU has so far lagged behind the U.S. with its sanctions, resisting imposing penalties that would target wide swaths of Russia's economy.

Europe's reluctance is in part the result of fears that sanctions on Moscow could boomerang back and hurt the continent's economy, which has close ties to Russia. However, given the large number of Europeans killed in last week's plane crash, U.S. officials believe the EU may be more motivated to deepen its penalties.

Obama said now is the time for Putin to "get serious" about resolving the crisis in Ukraine. If he fails to take steps to do so, Obama said the economic costs to Russia will continue to increase.

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 (Photo: AP Photo/J. Scott Applewhite)