There's word that Russian troops have now forced their way into a Ukrainian military base in Crimea.
A Ukrainian news agency says about 100 Ukrainian troops are stationed there, and they've barricaded themselves inside one of their barracks.
In the week since Russia took control of Crimea, Russian troops have been neutralizing and disarming Ukrainian military bases there.
Some Ukrainian units, however, have refused to surrender.
Meanwhile Russian president Vladimir Putin is standing firm on Ukraine after a phone call with President Obama.
After the hour-long call, Putin said Russia can't ignore calls for help from Russian speakers in Ukraine's Crimea.
He insisted Russia is acting in accordance with international law.