Ukrainian government forces are reported to have launched an operation in the city of Sloviansk. The city is a stronghold for pro-Russian separatists who are exerting increasing control over the eastern regions of the country. Separatists in the city and Russian television channels are reporting that the city is being stormed. News agencies report gunfire, explosions and a military helicopter firing on the outskirts of the city. The BBC's Sarah Rainsford, in eastern Ukraine, has spoken to Russian separatists who says fighting is taking place at checkpoints on the outskirts of the city - but she adds there is no...