A loud explosion has been heard in the Russian capital Moscow at a theatre where Chechen rebels are holding up to 800 people hostage. The blast occurred at 0915 local time (0515 GMT), but it was not clear whether it came from inside or outside or whether security forces had started to storm the building. It happened after overnight attempts to negotiate the release of the hostages appeared to have failed. Dozens of heavily armed Chechen rebels are holding the hostages at gun-point after invading the hall in the south of the city in the middle of a sell-out evening performance.