Ukraine's acting president says the crew members on two Ukrainian warships are" ready to defend their ships." 

Ukraine says the crew members have been given an ultimatum by Russian troops -- either surrender the ships, or they will be seized.

A Russian defense ministry spokesman is dismissing that report as nonsense, but he's refusing to elaborate. The ships are in a harbor in Ukraine's Crimea region, near Russia. Ukrainian officials say four Russian navy ships are blocking them. 

Russia today has effectively turned Crimea into a protectorate. Russian soldiers are in control of all Crimean border posts, as well as all military facilities in the territory. They also control a ferry terminal in a city that's just 12 miles across the water from Russia. 

As Russia tightens its stranglehold over Crimea, it's also calling for Ukraine to return to an agreement signed last month aimed at ending Ukraine's crisis. The agreement called for President Viktor Yanukovych to hold early elections and give up many of his powers. Yanukovych later fled to Russia. 

Secretary of State John Kerry is heading to Kiev, in a show of support for Ukraine's sovereignty. The European Union, meanwhile, is threatening a series of sanctions as it calls an emergency summit on Ukraine for Thursday. 

President Obama calls Russia's intervention in Ukraine a violation of international law and Ukrainian sovereignty.

The President says Russia is on the wrong side of history.  

The latest comments came as the U.S. considers possible sanctions against Russia for sending troops into the Crimean region, which Obama called "unprovoked."  

The President insisted that Ukrainians must be allowed to control their own destiny.