SEOUL, South Korea (AP) — 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 in a statement that authorities were mobilizing 55 rescue workers and 18 vehicles to rescue possible victims.

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

Gwangmyeong city 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.