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The city of Mariupol occupies a small area on the map, but it stands in the way of Russian forces who have burst out of the Crimean peninsula.

They are pushing north-east to try to link up with Ukrainian-separatist allies in the Donbas region of eastern Ukraine.

Capturing Mariupol is seen as vital to Russia's war effort.

Is Russia on the cusp on achieving one of its main strategic objectives?

Diplomatic Editor Mark Urban reports.