South Korean President Yoon Suk Yeol on Monday blamed authorities' failure to follow disaster response rules, Reuters reported on Monday.
Deluges have pummelled central and southern regions since Thursday as the rainy season that started in late June reaches its peak. The interior ministry has also reported nine people missing and 34 injured across the nation.
Twelve deaths, including three bodies found overnight, occurred in a tunnel in Cheongju, 110km (68 miles) south of Seoul, where 16 vehicles, including a bus, were swamped by a flash flood on Saturday after a river levee collapsed.
The incident fuelled questions over South Korea's efforts to prevent and respond to flood damage. Some drivers who use the road regularly blamed the government for failing to ban access to the underpass even though floods were widely forecast.
Yoon, just back from an overseas trip, on Monday convened a disaster response meeting and conceded the situation was made worse because of poor management of vulnerable areas.
"We've repeatedly emphasised access control over dangerous areas and preemptive evacuation since last year, but if basic principles of disaster response are not kept on the spot, it is difficult to ensure public safety," Yoon told the meeting.
Nearly 900 fire, police and military officials took part in the underpass rescue operation, using boats, underwater drones and other equipment, according to the interior ministry.
TM/PR