An explosion in a warehouse has sparked a major fire in Lagos, Nigeria's largest city and commercial capital.Witnesses said fireworks were stored in the warehouse in the busy market area of Lagos Island, and the blaze quickly spread to other buildings.The blast reportedly shook windows of homes several miles away and a thick cloud of smoke could be seen rising over the island.It was not immediately clear if there were any fatalities.An Associated Press journalist on site said Nigerian Red Cross workers had been treating people with light injuries.Thousands of people in the area gathered to watch, as the fire destroyed neighbouring buildings.The BBC's Tomi Oladipo, in Lagos, says emergency services have had problems reaching the scene of the fire because of the huge crowds."The firecrackers were packed in a shop, and the firecrackers caught fire," AFP news agency quoted a National Emergency Management Agency worker as saying.