(Adds quotes, criticism of Obama)
By Steve Holland
WASHINGTON, July 21 (Reuters) - U.S. President Barack Obama piled pressure on Russian President Vladimir Putin on Monday to force pro-Russian separatists in Ukraine to stop blocking an international investigation into the downing of a passenger jet last week.
In what has become a dramatic escalation in the worst East-West crisis since the Cold War, 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 the crash site and most of the bodies of the victims removed, Obama said Russia should compel the separatists to let the investigation go ahead.
He questioned why the rebels were blocking access. "What exactly are they trying to hide?" he said.
Obama did not specifically threaten new economic sanctions against Russia, and although senior Obama administration officials said additional sanctions were on the table, they made it clear fresh penalties were not imminent.
Adding sanctions now could interfere with the immediate priorities, which were to gain access to the crash site for an investigation and retrieve the bodies, they said.
CRITICS SAY OBAMA TOO WEAK
U.S. officials declined comment on whether Australia, which lost 28 people in the Malaysian plane, should rescind Putin's invitation to a G20 summit scheduled there in November. Prime Minister Tony Abbott is under some pressure to bar him entry. A senior U.S. official said this was a decision for the host country.
Some Republicans criticized Obama for being too weak toward Russia. Republican Senator Lindsey Graham tweeted: "The people who appreciated President Obama's speech the most were the Russians. Talks loud. No stick."
Nearly 300 people died when the Malaysian Airlines plane came down. U.S. officials have said it was their assessment that it was shot down by the separatists with a Russian-made SA-11 surface-to-air missile.
To bolster their case, U.S. authorities are considering releasing an unclassified intelligence assessment to provide as much material as possible tying the shootdown to the separatists with Russian-supplied hardware.
"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.
A senior administration official said a deal between Malaysia and the separatists to obtain the airliner's data-filled "black boxes" was welcome.
But the official said this did not obviate the need for a full-scale probe on site because the boxes only contain flight data and would not tell the full story of what brought the plane down. (Additional reporting by Jeff Mason and Roberta Rampton; Editing by David Storey and Cynthia Osterman)