Armed standoffs are taking place at several military bases across Ukraine's vital Crimea region.
Russian troops have taken over the area, saying it's for the protection of Russians who live there after Ukraine's pro-Russian president was ousted last week.
So far the troops have met with almost no resistance.
The Obama administration calls the invasion "an incredible act of aggression" but says a military response is not an option.
Instead, the U.S. and its allies are threatening Russian President Vladimir Putin with canceled trade deals, and economic sanctions.
Former CIA deputy director Mike Morell says President Obama's credibility is at stake.
“Things that we've done already are pretty weak, and Putin will see them as weak.” “The only thing that Vladimir Putin understands is tough,” said Mike Morell.
“Officials here at the United Nations are looking into options to stop the crisis from escalating. The Deputy Secretary General of the U.N. was sent to Ukraine last night to assess the situation,” said Alexis Christoforous.
Secretary of State John Kerry will meet with members of Ukraine's new government in the capital city of Kiev tomorrow.
The Obama Administration says it is ready to work with other countries to provide support for Ukraine's broken economy.