Among the latest developments related to the crisis in Ukraine:

-- "The United States and the European Union will respond on Monday with a 'serious series of steps' against Russia if a referendum on Ukraine's Crimea region goes ahead on Sunday, U.S. Secretary of State John Kerry said on Thursday. Kerry told a congressional hearing he hoped to avoid such steps, which include sanctions, through discussions with his Russian counterpart Sergei Lavrov, in London on Friday." (Voice of America)

-- "Russia has begun military exercises, involving more than 8,000 troops, close to the border with Ukraine. The defense ministry in Moscow confirmed that artillery such as rocket launchers and anti-tank weapons would also be involved in the exercises. They come at a time of high tension ahead of Crimea's referendum on Sunday on whether to join Russia." (BBC News)

-- "In an unusually robust and emotionally worded speech, German Chancellor Angela Merkel warned of 'catastrophe' unless Russia changes course. 'We would not only see it, also as neighbors of Russia, as a threat. And it would not only change the European Union's relationship with Russia,' she said in a speech in parliament. 'No, this would also cause massive damage to Russia, economically and politically.' " (Reuters)

Less ominous news: In what he said might be a "big step forward," the chairman of the Organization for Security and Co-operation in Europe announced that Russia now supports the idea of an OSCE mission in Ukraine, including Crimea, to monitor the situation. But, as Reuters also reported, an OSCE team that tried to enter Crimea on Thursday said it was turned away by armed men.

Need a refresher on what this crisis is all about?

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."

It was after Yanukovych left Kiev and headed for the Russian border that troops moved to take control of strategic locations in Crimea.

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