President Obama warned Monday that the United States has "serious concerns" about the increasing death toll among Palestinian civilians, as the death toll rose to more than 500 amid an Israeli military offensive in Gaza.

Speaking to reporters on the South Lawn, Obama reaffirmed his administration's support for the Israel's effort to thwart rocket attacks by Hamas militants. But the president cautioned that "we don't want to see any more civilians killed," and he called for Israel and Hamas to return to the terms of a 2012 ceasefire agreement. He said he had dispatched Secretary of State John F. Kerry to Cairo to help broker a diplomatic resolution to the latest fighting.

President Obama pauses while speaking about the situation in Ukraine during a news conference at the White House in Washington, July 18. (REUTERS/Jonathan Ernst)

Obama, in a hastily arranged appearance, also demanded that Russian President Vladimir Putin take immediate action to ensure open access for international investigators to the crash site of Malaysia Air Flight 17 amid reports that pro-Russian separatists in Ukraine have begun removing evidence from the area. U.S. analysts and other international experts believe the jet was shot down by a missile launched by separatist fighters and supplied by Russia.

"What exactly are they trying to hide?" Obama said of the separatists, who have denied responsibility for the attack on the plane. The president said that it is up to Putin to "insist the separatists stop tampering with evidence. There needs to be an immediate, full and unimpeded access to the crash site."

Obama added that there would be increasing consequences if Putin and Russia failed to change course in the ongoing fighting in eastern Ukraine.

"Russia will only further isolate itself from the international community," Obama said. "Now's the time for President Putin and Russia to pivot away from the strategy they've been taking and get serious about trying to resolve the hostilities in Ukraine."

However, Obama did not specify what consequences Putin would face. The United States last week ramped up additional economic sanctions against Russia's banking and energy sectors, but Obama has so far ruled out U.S. military action in the region.