BUJUMBURA (Reuters) - Somali rebels killed six Burundian peacekeepers during an offensive against insurgents, Burundi's army said on Thursday, one of the force's heaviest losses in a single day. The peacekeepers died in fighting in the Somalian capital Mogadishu on Wednesday. They were supporting Somali government troops who seized three militant bases along the city's industrial road that leads to the Bakara market, one of the main strongholds of the al Shabaab group.