Putin said he wanted to send troops to protect the lives of Russian citizens and military personnel based in the southern Crimea region, and normalize the "extraordinary situation in Ukraine."
Acting Ukrainian President Oleksandr Turchynov took to Twitter to defend the country's new government.
"We perceive Russia's actions as direct aggression towards the sovereignty of Ukraine," he wrote.
Ukrainian opposition leader Vitali Klitschko said he planned to ask Turchynov to call for an emergency Parliamentary meeting to vote to invalidate the Black Sea Fleet Naval Base agreement.
"We've made clear [to Russian officials] that they can be part of an international community's effort to support the stability and success of a united Ukraine going forward, which is not only in the interest of the people of Ukraine and the international community, but also in Russia's interest," Obama said.