A powerful natural disaster struck the South Korean city of Pohang on Wednesday, with local media reporting that resulting tremors were felt hundreds of kilometres away in Seoul.
Analysts were monitoring "unusual seismic activity" in North Korea, the head of the nuclear test monitoring agency CTBTO said.
Seven people were injured, Seoul's public administration ministry said, but they warned that number could rise because the quake was shallow.
The natural disaster took place some 9 kilometers from the coastal city of Pohang at 2:29 p.m.
Photos and video footage sent to local TV stations showed crumbled street walls, furniture violently shaking inside people's homes and people rushing out of buildings in panic.
Debris from a collapsed wall is scattered in front of a shop after an earthquake in Pohang South Korea
Emergency centres nationwide were flooded with thousands of calls seeking information, while Kakao Talk - the South's top mobile messenger application - reported service disruption due to heavy traffic.
The country's foreign exchange and stock markets will still open an hour late (0100 GMT) on Thursday, South Korean financial authorities said in text messages. An aftershock of 3.6 followed.
The university entrance exam is taken very seriously.
"We need to prepare for the possibility of a quake of 7 magnitude or higher, and bolster efforts to have more quake-resistant buildings and facilities around us".
Operations at nuclear reactors were not affected, the state-run nuclear operator Korea Hydro & Nuclear Power Co said. Nuclear power plants in Gyeongju and the surrounding areas are reported to be safe and unaffected by the natural disaster.
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