(CNN) -- The mood in Ukraine grew even more tense Monday as additional Russian troops poured into the Crimea region. Russia again defended its actions, saying it's protecting Russian citizens in a country that was on the brink of civil war. Meanwhile, world leaders opposed to Moscow's incursion plotted what to do next in the Ukraine crisis. What sanctions will there be? What kind of aid will Ukraine receive? Will there be fighting between Russian and Ukrainian troops?
In Crimea: Russia consolidated its hold on the peninsula. Russian troops arrived at a ferry crossing in Kerch, across a narrow sea channel where Ukrainian border guards reported seeing armored vehicles massing. The atmosphere remained calm, CNN's Ben Wedeman reported, with the troops milling about and clearly not on high alert. CNN's Diana Magnay calls it a 'low-key' invasion.
In Kiev: Tension pervaded the capital city Monday as residents awaited Russia's next move. Ukraine's political leaders appealed to world powers to help stop Russia and vowed to stand up to any further Russian moves. "Nobody will give Crimea away," interim Prime Minister Arseniy Yatsenyuk said. "There are no grounds for the use of force against civilians and Ukrainians, and for the entry of the Russian military contingent."
In Russia: Moscow denied a report in Russian state media claiming it had issued an ultimatum to Ukrainian forces to clear out of Crimea by Tuesday morning or face a "military storm." Meanwhile, Foreign Minister Sergey Lavrov brushed aside claims that Russia's troop movements were an act of aggression. "I repeat: This is a matter of defending our citizens and our compatriots, of defending the most important human right -- the right to life," he said.
In diplomacy: British Foreign Minister William Hague visited Kiev and U.S. Secretary of State John Kerry was heading there later to stand shoulder to shoulder with the country's shaky new government. In an interview with the BBC, Hague promised that Russia's decision to send troops to Crimea would bring "significant costs." The United Nations Security Council and European foreign ministers met Monday in an emergency session to discuss Ukraine. Senior U.S. officials tell CNN that the White House is quickly working on sanctions that would take the form of an executive order from President Barack Obama.
In the markets: Stocks fell across the world as investors chewed on the potential impacts of a ramped-up conflict in Ukraine. Russian stock indexes dipped as much as 13%. European stock markets were off sharply, as well -- as much as 3%. In the United States, the benchmark Dow Jones index dropped than 200 points.
How we got here: Last year, the former Ukrainian President's decision to scrap a European trade deal and turn toward Russia for aid inflamed anger in Ukraine's pro-European west and protesters took to the streets. Last month, those protests turned violent. A peace deal brokered by European foreign ministers evaporated into chaos as the President fled the capital, eventually turning up in Russia. Meanwhile, Moscow began warning of threats to ethnic Russians and pro-Russian Ukrainians, many of whom live in eastern Ukraine and in Crimea, where Russia has a major military base.
On Thursday, pro-Russian gunmen seized several government buildings in the Crimean capital of Simferopol. On Friday, mysterious troops appeared outside airports in Crimea. On Saturday, the Russian parliament gave President Vladimir Putin authority to send troops to Ukraine. Russian forces began flooding the peninsula.