Firefighters conduct a rescue operation following a suspected poisoning accident at the construction site in Paju, Gyeonggi Province, Thursday. Yonhap

More than 20 people have been injured from an apparent carbon monoxide poisoning accident at a construction site in Paju, just north of Seoul, fire authorities said Thursday.

A total of 21 workers were found injured at the apartment construction site as they were curing concrete, according to the authorities.

Among them, four were seriously injured and are being taken to the hospital, they added.

Authorities have dispatched 10 ambulances and are conducting a search within the site for additional casualties. (Yonhap)