More senior Russian officials are being added to the list of those who the U.S. will seek to penalize for their nation's interference in Ukraine's affairs, President Obama announced Thursday morning.

Without giving specifics, Obama said that additional officials who played a role in Russia's "illegal move" to annex Crimea will be subject to U.S. financial sanctions and travel bans. According to the Treasury Department, 20 individuals are covered by the new sanctions. Their names are listed here. Also on the list: one financial institution, Bank Rossiya.

Earlier, sanctions were aimed at 11 individuals — most of them Russians, but also including former Ukrainian President Viktor Yanukovych.

Obama also warned that the U.S. is prepared to, if necessary, impose sanctions on "key sectors of the Russian economy," a step that could further ratchet up tensions between the two nations.

"Nations do not simply redraw borders or make decisions at the expense of their neighbors simply because they are larger or more powerful," Obama said, and "Russia must know that."

This latest development related to the crisis in Ukraine follows the dramatic steps taken this week by Russian President Vladimir Putin, who moved to absorb Crimea into the Russian Federation after citizens there voted to break away from the rest of Ukraine.

Russian troops, along with local self-defense forces, moved to take control of Crimea shortly after last month's toppling of Ukrainian President Viktor Yanukovych, an ally of Putin's.

Background

As the crisis in Ukraine has developed over recent weeks, we've tracked developments. Here's a recap:

Crimea has been the focus of attention as the ripple effects of the protests that led to last month's ouster of 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

Shortly after Yanukovych was deposed and fled Ukraine, Russia moved to take control of Crimea by sending thousands of troops there to secure strategic locations. Along with "local defense forces," those soldiers surrounded Ukrainian military facilities.

This week, after Crimeans voted to join the Russian Federation, Russian President Vladimir Putin signed a treaty to annex the region. The U.S. and European Union have objected, calling that action a violation of international law. Putin says he is supporting Crimeans' right to "self-determination."

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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