A subway tunnel under construction near Seoul collapsed on Friday, likely leaving two workers trapped at the site, officials said.

The National Fire Agency said authorities were mobilising 55 rescue workers and 18 vehicles to rescue possible victims.

The collapse happened at Gwangmyeong, a city just south of Seoul.

The city's officials said authorities earlier withdrew workers from the construction site and stopped traffic around the area after receiving reports that a ventilation shaft at the site was at a risk of collapse.

The officials said it wasn't immediately known whether any workers re-entered the site. (AP)