Update: President Obama addressed the ongoing Ukraine crisis later on Monday, telling reporters that Russia broke international law with its military presence in Crimea. He added that Russia's mobilization of troops had put the country "on the wrong side of history." The president also indicated that the U.S. was considering sanctions against Russia: "if in fact they continue on the current trajectory, that we are examining a whole series of steps -- economic, diplomatic -- that will isolate Russia."

Original Post: According to multiple reports, Russia issued an ultimatum to Ukraine's forces in Crimea: surrender or face a military response. But Russia is disputing that report as "nonsense," telling Russia Today that the first they'd heard of the threat was in the "media."  The alleged threat comes after Russia's military managed to take over control of Crimean border posts and military facilities.

Whether it's true or not, the alleged ultimatum has ratcheted up existing tensions over the Russian military's presence in Crimea. A spokesperson for the Ukrainian defense ministry told the AP on Monday that four Russian ships were blocking two Ukrainian war ships in the Black Sea, presumably waiting for an answer to the alleged ultimatum. Despite the Russian denial, some reporters at the scene of the stand-off between Russian and Ukrainian ships say they heard the Russian fleet issue Ukraine a deadline: 

Meanwhile, Russia is defending, rather than disputing, its overwhelming presence in CrimeaRussian Foreign Minister Sergey Lavrov said that the troop presence would continue "until the normalization of the political situation,” and that Russia needed military control of the region for the purpose of "defending our citizens and compatriots, ensuring human rights, especially the right to life."  

As Ukraine's new interim government calls Russia's advances an act of "war," leaders around the world are scrambling to get each other on the phone to come up with some sort of diplomatic response to the crisis. Russian Prime Minister Dmitriy Medvedev and Vice President Joe Biden spoke earlier on Monday, during which Biden "urged Russia to pull back its forces, support the immediate deployment of international monitors to Ukraine, and begin a meaningful political dialogue with the Ukrainian government," according to the White House. And Russia requested an emergency meeting of the UN Security Council, to convene later in the afternoon on Monday.