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In a symbolic White House meeting with Prime Minister Arseniy Yatsenyuk of Ukraine today, President Obama said the U.S. and the international community reject the "slap-dash referendum" on the future of Crimea and warned Russia not to deal with Ukraine "with the barrel of a gun."

Yatsenyuk urged Russia to pull back its forces and engage in diplomatic negotiations.

"Mr. Putin, tear down this wall," he said, as he emerged from the White House, "the wall of war, intimidation, and military aggression. Let's talk."

Yatsenyuk's quote played off President Reagan's famous Cold War-era call for Soviet leader Mikhail Gorbachev to tear down the Berlin Wall that divided the eastern and western blocs.

Today's meeting, also seen as a message to Moscow, came just days before residents in the pro-Russian Crimean Peninsula will vote to decide whether to become part of Russia.

"We completely reject a referendum patched together in a few weeks with Russian military personnel basically taking over Crimea," the president told reporters following an Oval Office meeting with Yatsenyuk. "We reject its legitimacy. It is contrary to international law. It is contrary to the Ukrainian constitution.

"The current government in Kiev has recognized and has communicated directly to the Russian Federation their desire to try to manage through this process diplomatically," Obama said. "They cannot have a country outside of Ukraine dictate to them how they should arrange their affairs, and that there is a constitutional process in place and a set of elections that they can move forward on that, in fact, could lead to different arrangements over time with the Crimean region. But that is not something that can be done with the barrel of a gun pointed at you."

Obama continued to warn Russia that there will be costs if it does not back down.

"We will continue to say to the Russian government that if it continues on the path that it is on, then not only us but the international community, the European Union and others will be forced to apply a cost to Russia's violations of international law and its encroachments on Ukraine," he said.

"There's another path available, and we hope that President Putin is willing to seize that path," he added. "But if he does not, I'm very confident that the international community will stand strongly behind the Ukrainian government in preserving its unity and its territorial integrity."

Following the meeting with President Obama, Yatsenyuk spoke at the Atlantic Council, where he said he and Obama had a "very frank and open" discussion about the crisis and what the Ukrainian government needs. He said he was "satisfied" with the response from the United States and Europe and commended the American people for their support.

He minced no words about where the Ukraine stands on Russia's incursion into Crimea.

"It's all about freedom," said Yatsenyuk. "We want to be very clear. We will never surrender."

He maintained that the upcoming referendum on Crimea independence is illegal, and disputed Russian arguments that the Ukrainian government cannot support Crimea or protect Russian supporters.

"Crimea is an integral part of Ukraine," he said. "This is our territory and these are our citizens."

Calling the military action in Crimea by Russia a threat to global security, Yatsenyuk warned that once "the bloodshed" begins in the peninsula, it may never end.

He also had harsh words for President Putin, who he said could be trying to take over all of Ukraine. Yatsenyuk referenced a speech Putin made years ago where the Russian leader said the biggest disaster of the last century was the destruction of the Soviet Union.

"The biggest disaster of this century would be the restoration of the Soviet Union," he said.