Hence, evacuation centres were being prepared in the city on Monday, amidst mounting tensions.
There are 3919 emergency shelters across Seoul - many are housed in subway stations or underground carparks.
According to local officials, evacuation shelters can be easily reached by everyone within five minutes and can accommodate more than 20 million people.
Each subway station has gas masks stored in glass cabinets and are called "nation masks".
SOS emergency telephones directly connect people to the emergency control room.
The maintenance area for subway stations have been fixed with machinery to provide clean air in case of an attack.
However the government is concerened about the fact that many Seoul residents are still unaware of emergency procedures or even the existence of shelters.