Kaohsiung lies adjacent to a huge petrochemical complex housing dozens of plants and many pipelines run under the densely packed city.
The explosions were the second disaster to strike Taiwan in just over a week, after a TransAsia Airways plane crashed with the loss of 48 people on July 23.
Government spokesman Sun Lih-chyun told AFP the Taiwanese flag would be flown at half-mast from public buildings for three days of national mourning for "our compatriots who died unfortunately in the two accidents".
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