BLITZER: We'll return to our breaking news coverage of the mystery of Flight 370 in a moment. But, first, this urgent story. The Obama administration expressing concern about what it sees as a potentially imminent Russian incursion into Ukraine. A source telling CNN the United States is closely monitoring Russian troops amassing on the Ukrainian border. Russia says troops are there for military exercises. But U.S. officials say there's reason to be skeptical. Our senior international correspondent Nick Paton Walsh is joining us from Crimea with the latest -- Nick.</s>NICK PATON WALSH, CNN SENIOR INTERNATIONAL CORRESPONDENT: Wolf, a day for tit-for-tat</s>BLITZER: Nick Paton Walsh, thank you. Here's a recap of the very latest developments on the mysterious disappearance of Flight 370. Among the latest: Australian officials say six planes will scour the Indian Ocean about 1,500 miles southwest of Perth, looking for debris from the missing plane. Also, Britain's telegraph newspaper has obtained what it says is a transcript of the final 54 minutes of communication between the cockpit and air traffic controllers. And the Pentagon says it's looking at what underwater surveillance equipment it can use to help with the search. That's it for me. Thanks very much for watching. I'm Wolf Blitzer in Washington. "ERIN BURNETT OUTFRONT" starts right now.
20K Russian Troops Massing Near Ukraine Border