(This post was updated at 4:52 p.m. ET. to reflect President Obama's meeting with the interim prime minister of Ukraine.)

Conceding that "we cannot launch a military operation in Crimea, as we would expose the eastern border and Ukraine would not be protected," Ukraine's acting president has told Agence France-Presse that his nation won't use force in a bid to keep Crimea from breaking off and joining the Russian Federation.

Oleksandr Turchynov also writes on the op-ed pages of The New York Times on Wednesday that "no one should doubt that Ukrainians are prepared to defend their country." But, he says, "the memory of our people's terrible losses during the [recent] protests in Kiev is still fresh; we cannot permit more bloodshed."

"We are fully aware that," Turchynov adds, that "should force be used, containing the situation would be impossible."

Turchynov's words come as Ukraine's interim prime minister, Arseniy Yatsenyuk met with President Obama this afternoon at the White House.

Appearing with Yatsenyuk, Obama said the U.S. will "stand with Ukraine" and was prepared to apply a cost to Moscow if it doesn't change course in Crimea.

Yatsenyuk for his part said Ukraine "will never surrender" to Russia.

Turchynov's words also come as the pro-Russia leaders in Crimea — an autonomous region of Ukraine that has historically long ties to Russia and where Russia maintains a naval base — prepare for a public referendum on Sunday. The question they're putting before Crimeans: Should the region break away and join the Russian Federation. That referendum is expected to pass.

The AP reports Obama expressed hope the vote could be stopped. But if it is not stopped he said the U.S. won't recognize the result.

As we've previously said, Crimea has been the focus of attention as the ripple effects of the protests that led to last month's ouster of Ukrainian President Viktor Yanukovych have spread.

Summing up the history and importance of Crimea to Russia and Ukraine isn't possible in just a few sentences, of course. The Parallels blog, though, has published several posts that contain considerable context:

-- Crimea: 3 Things To Know About Ukraine's Latest Hot Spot

-- Crimea: A Gift To Ukraine Becomes A Political Flash Point

-- Why Ukraine Is Such A Big Deal For Russia

We've recapped what set off months of protest in Kiev and ultimately led to Yanukovych's dismissal by his nation's parliament last month this way:

"The protests were sparked in part by the president's rejection of a pending trade treaty with the European Union and his embrace of more aid from Russia. Protesters were also drawn into the streets to demonstrate against government corruption."

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