Landmines planted by the airforce in the 1960s could have slipped down the Wumyeon mountain hit by a mudslide in southern Seoul, the defence ministry said.
Soldiers were waiting for the rain to stop to begin searching, officials said.
Most of the landmines planted around an air defence base on the mountain were removed between 1999 and 2006 but 10 remained unaccounted for and officials fear they could be caught up in the mud which has been displaced by the torrential rains.
Fears of landmines in the north of the country have also prompted the Joint Chiefs of Staff to order search operations where needed.
About 5,000 firefighters, soldiers, police officers and other rescuers were mobilised on Thursday as the estimates of dead and missing hit 67, according to the Yonhap news agency.
Rainfall stopped or decreased in many parts of Seoul and the surrounding area on Thursday but weather forecasters said that it will continue until Friday in parts of the country. Heavy rain has also hit North Korea.
Most highways were reopened on Thursday but 32 roads and bridges remained closed.
The government announced that it will strengthen guidelines for mountains and steep slopes in the wake of the massive downfall. Victims are to be helped by emergency funds and an emergency response system set up to respond to extreme weather conditions.