A loud explosion shook central Baghdad late on Thursday, sending a pall of oily black smoke into the night sky and flames could be seen from a kilometre (half a mile) away. One police source said an initial report indicated that it may have been caused by exploding gas canisters close to an electricity sub-station. Reuters correspondents in the city heard gunfire coming from the nearby Karada district after the blast. Earlier in the day, three suicide bombers were thwarted in an attack on the Green Zone government compound in the city. --SP 2257 Local Time 1957 GMT