Rescue workers on Wednesday retrieved the body of one of the six workers who went missing at a flooded underground waterworks construction site in Seoul two days ago, firefighting authorities said.

The body was identified as that of a 48-year-old Chinese worker named Park Myeong-chun and was moved to a nearby hospital, they said.

On Monday night, water from the swollen Han River swamped the construction site, killing one worker and leaving six others unaccounted for. Torrential monsoon rains have pounded the capital city and adjacent areas in recent days.

Police and the firefighting authorities sent four teams of divers into the flooded site in search of the missing workers early Wednesday.

The divers then found the body near the mouth of a 1.5-km-long underground tunnel where the workers are suspected to be trapped, the authorities said.

At the time of the flooding, the ill-fated workers were removing old water pipes inside the tunnel 25 meters below ground, they said.

The authorities expect that the bodies of the five other missing workers will be recovered during the day as the level of water has significantly dropped after overnight drainage.

Seoul Mayor Park Won-soon visited the scene of the incident again on Wednesday, making a public apology.

"I deeply feel responsible for failing to protect the lives and safety of citizens and apologize for this," Park said.

He vowed a thorough investigation into the cause of the incident and to come up with measures to prevent any recurrence.

The mayor previously visited the scene on Monday night when the accident occurred.

Heavy rains have battered mostly northern parts of South Korea in recent days, including Seoul and surrounding Gyeonggi Province, killing three other people and displacing 200 others. (Yonhap News)