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Ukraine Ambassador to the United Nations Yuriy Sergeyev, in an interview with Fox News, warned that "there is a threat of war" and demanded the Russian occupation of Crimea end immediately.

But despite Russian forces streaming into Ukraine's Crimea Peninsula, Sergeyev vowed: "We will succeed." Speaking to Fox News, he called on world leaders to stand up against the Kremlin's aggression, "to speak to Putin and the Russian leadership to cool them down." Sergeyev accused the Russians of breaking international law.

"They violated all the principles of the United Nations, the principles of the Security Council -- that is why they have to answer the world about what they are doing," he said.

Sergeyev addressed the United Nations Security Council on Saturday, but his pleas for international intervention on that front seemingly have little chance of success in light of the certainty of a Russian veto. Russian Ambassador Vitaly Churkin dismissed his Ukrainian counterpart's accusations and blamed the West for stirring up anti-Russian sentiment that led to the crisis.

With Crimea's TV station and airports under Russian control and fresh memories of Russian forces invading and supporting the breakaway regions of Abkhazia and South Ossetia in the former Soviet republic of Georgia, Sergeyev raised concerns of  Moscow putting Crimea and Eastern Ukraine under Russian control.

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    "This is an act of aggression, under all United Nations provisions," he says. "We have to stop the expansion of this aggression."

    When Fox News asked about the prospect of war, the ambassador sighed deeply in apparent concern and frustration.

    "I am trying to pray and to not even think about that but we have to be prepared for what happens," he said. "We are preparing to defend ourselves. We are preparing to defend ourselves with the help of our partners, so the strong message is resounded in Europe and the United States."

    Russian President Vladimir Putin has justified the troop movements as necessary to protect the Russian and Russian-speaking population of Crimea, a claim the ambassador dismisses as an excuse. He notes that under such thinking, Putin could use the pretext of protecting Russian-speaking New Yorkers who live in the Russian community of Brighton Beach, Brooklyn, to invade the United States. He says the goal for the Ukrainians is not to give Moscow any reason to try and claim that their invasion is legitimate.

    "Ukrainian military men and police in Crimea, they are trying to be cool and not ... provoke any conflict, not to give the motivation for Russia to prove that they are exactly there because of the violations. No violations, no violations there, that is why we are still trying to find a diplomatic solution, still inviting them to the dialogue, still asking the world leadership to stop the threat of Russian expansion and a possible war."

    Sergeyev, who was a professor and academic before entering the Ukrainian diplomatic service after the fall of the Soviet Union, says that he, and the Ukrainian people, do not want to go back to the days of being dominated by Moscow. He says his nation has cherished its freedoms and democracy, which are now threatened yet again.

    "I absolutely share what people in our streets are demanding. They do not want to come back. They do not want to live in the conditions that reminds us of the past in the Soviet Union, the corruption and the domination of the political figures."

    Fox News' Jonathan Wachtel contributed to this report.