The ninth floor of the nine-story building under construction in Anseong, some 60 km southeast of Seoul, collapsed onto the eighth floor at about 11:49 a.m. local time (0249 GMT), Yonhap news agency reported.
The accident left two Vietnamese workers buried and four others wounded.
The Vietnamese men were rescued in cardiac arrest before being taken to a hospital, but they were confirmed dead later.
AMK/PR