The Russian economy might lose billions of dollars if the situation is not reversed, said former US ambassador to Russia Michael McFaul.

      According to him, the US has an emergency scenario if the tension is escalated but he did not disclose the details.

      Washington has boosted its military presence near Ukraine's borders, he told the Voice of Russia.

      Earlier, Secretary of State John Kerry issued a diplomatic ultimatum to his Russia counterpart, Foreign Minister Sergey Lavrov, according to a senior U.S. administration official.

      "He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy, and he urged utmost restraint," the official was quoted by CNN as saying.

      Kerry said the United States is ready to work with allies to facilitate a Ukraine-Russia dialogue, the official said.

      Earlier, Russia-IC reported that the U.S. is prepared to back up NATO militarily if the unrest in Ukraine escalates,

      "We're trying to tell [Russia] not to escalate this thing further into Eastern Ukraine, and allow the conditions to be set for some kind of resolution in the Crimea," Joint Chiefs Chairman Gen. Martin Dempsey said in an interview with PBS Newshour's Judy Woodruff.

      When asked if there is a chance of U.S. military intervention in the Ukraine crisis, Dempsey replied, "That's a question that I think deserves to be assessed and reassessed and refreshed as this thing evolves."

      Dempsey cautioned, "If Russia is allowed to do this, which is to say move into a sovereign country under the guise of protecting ethnic Russians in Ukraine, it exposes Eastern Europe to some significant risk, because there are ethnic enclaves all over Eastern Europe and the Balkans."

      The US ups the ante after diplomatic negotiations have failed to produce a solution to the unfolding crisis in Ukraine after President Yanukovich was ousted from power following months of protests.

      Russia insists Ukraine’s interim authorities need to come back to the Crisis Settlement Agreement signed on February 21 in the presence of international mediators.

      The West has backed the new regime and put pressure on Russia to stop meddling in Crimea.

      Follow Russia-IC for more news on the Ukraine crisis.