Obama warns Putin after Oval Office visit from Ukraine's leader

President Obama said Wednesday that the United States would be “forced to apply a cost” if Russia does not back away a secession referendum in the occupied Ukrainian territory of Crimea that is scheduled for Sunday.

Hosting Ukrainian prime minister Arseniy Yatsenyuk at the White House, Obama he and other global leaders "completely reject" the secession vote as a "violation of international law."

He called the threat to Ukraine's territorial integrity and sovereignty "the most pressing challenge" faced by the country, and said the referendum vote had been "patched together in a few weeks."

“There’s another path available and we hope [Russian President Vladimir] Putin is willing to seize that path,” Obama said. “But if he does not, I’m very confident that the international community will stand firmly behind the Ukrainian government.”

The president stressed that the Ukrainian government was willing to negotiate with the Kremlin to assure its interests in the region were protected. Russia has a naval base on the ethnically Russian Crimea peninsula, and has voiced concerns that Russian nationals could be persecuted by the new, pro-European Ukrainian government.

“But that’s not something that can be done with the barrel of a gun pointed at you," Obama said.

Obama praised Yatsenyuk, who he said "showed tremendous courage" in the fight for democracy in Ukraine.

Yatsenyuk, for his part, said Ukrainian leaders would "never surrender" in their fight for independence.

"We urge Russia to stick to its international obligations, to pull back its military … and to start the dialogue with no guns, with no military, with no tanks, but with the diplomacy and political tools," Yatsenyuk said.

The Ukrainian leader told reporters after the meeting that it was "absolutely unacceptable to have Russian boots on the Ukrainian ground."

"We do believe that in the near future the new Ukrainian government will be ready to deliver real changes. But in order to deliver those changes we need to stop the Russian military," he said.

Yatsenyuk said he feared Putin aspired to move forces further into Ukraine and "revise the outcomes of the Second World War."

“Mr. Putin, tear down this wall, the wall of war, intimidation and military aggression. Let's talk," he said.

In the Oval Office meeting, President Obama also reiterated his call on Congress to "act promptly" on a $1 billion loan guarantee package to help stabilize the Ukrainian government.

"I would just ask both Democrats and Republicans, who I know are unified in their support of Ukraine, to move quickly to give us the support that we need so that we can give the Ukrainian people the support that they need," Obama said.

The meeting between Obama and Yatsenyuk came ahead of a meeting between Russian Foreign Minister Sergey Lavrov and Secretary of State John KerryJohn Forbes KerryOvernight Defense: Pompeo clears Senate panel, on track for confirmation | Retired officers oppose Haspel for CIA director | Iran, Syria on agenda for Macron visit Senate must save itself by confirming Mike Pompeo Pompeo faces pivotal vote MORE scheduled for Friday in London.

Kerry is hoping to broker talks between the Ukrainians and Russians in a bid to deescalate the crisis.

White House press secretary Jay Carney said Kerry would "make clear that there is a way out of this diplomatically and peacefully that Russia can avail itself of."

"To continue that discussion is certainly worth doing, in our view, because there are already costs associated with Russia's decisions here, its violations, and there will be greater costs inevitably if Russia continues down this path," Carney added.