Four workers have been killed in an explosion at the STX Offshore & Shipbuilding plant in Changwon, South Gyeongsang Province, fire authorities said Sunday.

The blast occurred at around 11:37 a.m. when workers were doing painting works inside of a 12-meter-deep tank of a 74,000-ton oil carrier.

STX Offshore & Shipbuilding, a Korean shipyard company focusing on small- and mid-sized ship constructions, has been schedule to deliver the carrier to a Greek shipping company Andriaki Shipping by October.

The deceased were identified as employees hired by a subcontractor company. They were each aged 33, 45, 52 and 53.

The fire squad was dispatched to the accident site and entered the interior of the tank at around 12:04 p.m., upon report of the blast that was followed by smoke.

The carrier was anchored at some 250 meters from the front gate of the plant and no additional casualties were found, the authorities said.

The local police and fire authorities said they are currently investigating the details of the accident including the cause.

The workers were donned in goggles, dustproof clothing and other protective gear, company officials were quoted as saying.

Painting inside an enclosed space entails high risk of an accident in the shipyards due to oil mist created during the process that could cause an explosion or suffocation, according to the industry. Strict maintenance and safety regulations are thereby applied, such as by measuring the oxygen density and applying tools and clothing with explosion-proof functions.

The company reportedly explained one of three of the safety maintenance managers who came to work on the day gave the go-ahead for the painting job upon normal procedure and that no inflammable-related operation was being conducted in the vicinity.

By Shim Woo-hyun (ws@heraldcorp.com)