WASHINGTON - The United States is preparing to impose sanctions on Russia for its military intervention in Ukraine's Crimea region although no decisions have yet been made, the US State Department said on Monday.In her daily briefing, US State Department spokeswoman Jen Psaki also said that if reports that Russia has threatened Ukraine's military in the Crimea with attack are true, this would be a "dangerous escalation" of the situation. Ignoring warnings from US President Barack Obama, Russian President Vladimir Putin won permission from his parliament on Saturday to use military force in Ukraine. The stated purpose was to protect ethnic Russians following the ouster of Ukraine's Russian-backed president a week ago.Putin got the green light from parliament after Russian forces had already gained control of Crimea, an isolated Black Sea peninsula with an ethnic Russian majority and where Moscow has long had a naval base.US Secretary of State John Kerry, who is traveling to the Ukrainian capital Kiev on Tuesday in a show of solidarity, on Sunday publicly raised the possibility of economic sanctions such as asset freezes and visa bans on Russian individuals.His spokeswoman on Monday suggested that the administration was moving closer to sanctions, though she said no decisions had yet been made."At this point we are not just considering sanctions given the actions Russia is taking, and it is likely we will put those in place and we are preparing that right now," Psaki said. "We have a broad range of options available.""We are looking at the best way to hold people accountable. Obviously we will make those decisions and decisions will be made at a high level, but we are preparing options and we are likely moving down that path if things proceed," she added.US President Barack Obama said on Monday that Russia has violated international law in its military intervention in Ukraine and said the US government has warned it will look at a series of economic and diplomatic sanctions that would isolate Moscow.Russian President Vladimir Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Obama told reporters before he met with Prime Minister Binyamin Netanyahu."Over time this will be a costly proposition for Russia. And now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force," Obama said. Earlier on Monday, Russia's Interfax news agency quoted an unnamed source in the Ukrainian defense ministry saying the Russian Black Sea Fleet's commander had set an 0300 GMT deadline for Ukrainian forces in the region to surrender.The news agency later issued reports saying no such ultimatum had been issued.Asked about the reported ultimatum, Psaki told reporters: "These reports today of threats of force against Ukrainian military installations would, if true, in our view constitute a dangerous escalation of the situation for which we would hold Russia directly responsible."