Ukraine crisis: Gas supplies threatened over debt to Russia

Russia's state gas company Gazprom has threatened to cut off supplies to Ukraine if it fails to pay in advance for its June supplies.

The warning comes as relations between the two countries remain tense.

Fighting has intensified around Sloviansk in eastern Ukraine, with 12 troops killed on Thursday when pro-Russian rebels shot down an army helicopter.

Aaron Heslehurst explains how Ukraine has run up a $5.2bn (£3.1bn) bill.