Fire officials said most of the victims were construction workers trapped inside the basement of the refrigerated goods facility who could not escape a toxic cloud of gas that came from burning insulation material.
The exact cause is not yet known but local media said the authorities were investigating whether gas caught fire in a basement engine room at the warehouse, starting the blaze.
Ten workers were taken to hospital and another seven escaped without major injuries, fire officials said.
A doctor at the Bestian medical centre in Seoul, which specialises in burns treatment, said four of the injured were in critical condition in intensive care with burns to more than 40 per cent of their bodies.
More than 600 fire fighters and police were dispatched to the scene.
|Most of the victims were so badly burnt, they|
could not be immediately identified [AFP]
The victims included 12 ethnic Koreans who are Chinese citizens, the Chinese embassy in Seoul said.
The doctor at the Bestian medical centre said one of the injured was an ethic Korean from China.
China has a sizeable ethnic Korean minority and in recent years some have migrated to South Korea to seek work.
Like other foreign migrants, they often end up working in factories or doing difficult and low-paying work that affluent South Koreans have increasingly shunned.
The fire is the deadliest in South Korea since a blaze in the subway system of Daegu, 250km southeast of Seoul, killed nearly 200 people in 2003.