The tanker had broken down along the busy Onitsha-Owerri Road, in front of the Relief Market.
Shops and building around the accident site are feared to be burnt as a result of the fire explosion.
The fire which started at 8.30am had caused a long standstill of vehicles in and out of Onitsha.
Fire service both from Anambra and Delta states have arrived the scene and are battling to quench the fire amid a shortage of water.