evidence: Most recent fighting has centred on Heglig - and the spread of fighting further along the border is of great concern, says the BBC's James Copnall in Khartoum. [...] The UN and the African Union have demanded South Sudan's unconditional withdrawal from Heglig, calling its occupation "illegal and unacceptable", but they have also condemned Sudan for carrying out aerial bombardments of the South. [...] Heglig, which used to provide more than half of Sudan's oil, is internationally accepted to be part of Sudanese territory - although the precise border is yet to be demarcated.