(CNN) -- It's been building for months. And now, according to some, Russia has launched a "full-scale invasion" of Ukraine. U.S. officials say Russian troops were directly involved in the latest fighting, alongside pro-Russian rebels. "Russia is responsible for the violence in eastern Ukraine. The violence is encouraged by Russia. The separatists are trained by Russia; they are armed by Russia; they are funded by Russia," President Barack Obama told reporters Thursday. Moscow, meanwhile, has said it would do everything possible to protect ethnic Russians in eastern Ukraine, but denies direct involvement in the fighting. It says the United States must stop interfering.
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The United States and allies blame Russia for the violence
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Moscow denies direct involvement in the fighting
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Diplomacy and sanctions must play a role, experts say
At one point, a security camera at @placeholder military base captured a bizarre sight -- a Russian armored personnel carrier busting through a base gate.
There have also been sanctions that sought to freeze any U.S. assets in @placeholder and prohibit American business contacts for eight Russian arms companies that make weapons.