As tensions with Russia continue to rise in eastern Europe, CNN gains unprecedented access to the Ukrainian president on the front line in Donbas, eastern Ukraine. Ukrainian military officials tell CNN they estimate more than 50,000 Russian troops are massing near the border. CNN's Matthew Chance reports.
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Ankle-deep in thick black sludge, Ukrainian President Volodymyr Zelensky moves stealthily with his troops in single file through the warren of trenches and tunnels that form the tense front lines in the east of his country.