Among those killed were several firefighters.
Republic of China Premier Jiang Yi-huah, at the scene of the incident, said gas leaks in the sewage system of the city's Cianjhen District were the cause.
Officials believe the death toll could climb further because the explosion caused a road to collapse and victims may have fallen into openings.
Firefighters from Kaohsiung County and adjacent counties, as well as hundreds of soldiers, are involved in rescue operations. The cause of the gas leaks has not yet been established, Taiwan's Central News Agency said.
Kaohsiung City is on southwestern part of the island of Taiwan and its second-largest largest city, after Taipei, with a population of 2.77 million.