Referring to Russia's actions in Crimea as an "intervention" and saying the U.S. will continue to "mobilize the international community to condemn this violation of international law," President Obama just delivered some of his most extensive remarks to date about the crisis in Ukraine.

In laying out his rationale for ordering the Treasury and State departments earlier today to begin imposing travel and financial sanctions on individuals who are responsible for Russia's actions, Obama added that the world is "well beyond the days when borders can be redrawn over the heads of democratic leaders."

While Russian President Vladimir Putin denies that they are Russian forces, armed soldiers who witnesses and journalists say are Russian troops have effectively taken control in Crimea, a strategically important peninsula where Moscow has a naval base. The soldiers moved in after Ukrainian President Viktor Yanukovych was ousted by his nation's parliament — a toppling that came after months of protests over widespread corruption and Yanukovych's pro-Russian leanings.

He decried the move by the parliament in Crimea, an autonomous region of Ukraine, to take steps toward breaking off and joining Russia. "The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," Obama said.

There is a way to resolve the crisis that respects Russia's interests, the president said: "Let international monitors into all of Ukraine, including Crimea ... to ensure the rights of all Ukrainians are being respected."

Update at 6:45 p.m. ET:

The White House says Obama and Putin spoke for about an hour on Thursday. Obama reportedly re-emphasized that Russia's actions are a violation of Ukrainian sovereignty and that Moscow needs to move its troops back to their military base in Crimea in order to resolve the conflict.

Update at 4:05 p.m. ET:

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