SEOUL (AFP): At least two people were trapped Friday when a construction site for a new metro line collapsed near the South Korean capital Seoul, the National Fire Agency said.

The site was part of an underground transit project connecting Seoul's Yeouido district to Ansan and Siheung in Gyeonggi province, the country's most populous region.

Two people have been confirmed trapped, but further details are still being verified, the agency said in a statement.

One of the two has been located and was being rescued, but the other -- an excavator operator -- remains missing.

The National Fire Agency added that 55 firefighters had been deployed for the rescue operation.

The site had already been evacuated due to safety concerns early Friday morning, before the collapse, and police had closed off a one-kilometre (0.6-mile) stretch of road around the construction area.

A police official told local media that a site supervisor had reported cracks in an underground support column, which led to a joint response with city authorities.

"There were concerns over possible ground subsidence, so we blocked the road as a precaution," the official told reporters.

Police said they would keep the road closed until the reinforcement work is finished and the site is deemed to be safe.

The incident comes just weeks after one person was killed when a massive sinkhole opened up in Seoul.

The sinkhole accident took place at a site where extension work for a different metro line was underway. - AFP