Pifer says, historically, progress on arms control has led to advances in other areas with the Russians.

皮斐尔说，从历史上看来，武器控制上的顺利进程能带来与俄罗斯其它领域方面的进步。

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Senior producer Jennifer Pifer and associate producer Sabriya Rice contributed to this report.

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Pifer says Ukraine and its international supporters should be wary of any Russian efforts to revive separatist feelings in Crimea.

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CNN's Miriam Falco, Jennifer Pifer and Sabriya Rice contributed to this report.

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