At least 38 people have died and 10 more are in hospital after a fire broke out at a construction site in South Korea.

The blaze began on a building site of a warehouse in the city of Icheon, about 80 kilometres south-east of Seoul.

What started the inferno is currently unknown. Investigators suspect it may have been triggered by an explosion while workers were operating in an underground level of the site.

Rescue workers in white protective suits were seen carrying out victims in body bags.

"There was no clothing left (on the workers) at all," said Seo Seung-hyun, head of the Icheon fire department.

Mr Seo said victims who died on other floors likely inhaled toxic gas. He said more than 400 firefighters and rescue workers and about 120 vehicles were deployed to the scene.

Officials said eight construction workers were taken to hospital for treatment of serious injuries, while two others were slightly hurt. About 30 workers managed to escape but at least one remains unaccounted for.

South Korean President Moon Jae-in called for officials to mobilise "all available resources" in the search and rescue operation, while Prime Minster Chung Sye-kyun visited the site to inspect the efforts.

South Korea, one of Asia's richest economies, has struggled for decades to improve safety standards and change widespread attitudes that treat safety as subservient to economic progress and convenience.

Icheon was the site of another massive fire in 2008 when 40 workers died at a refrigerated warehouse.

46 people died in 2018 when a fire ripped through a small hospital with no sprinkler systems in the southern city of Miryang.