The quake struck four kilometres south of the county of Buan, which is around 205 kilometres south of Seoul, at 8:26 am (1126 GMT), according to the Korea Meteorological Administration.

An alert text message was sent to citizens nationwide, which advised those in affected areas to be cautious of possible falling objects and aftershocks.

Prime Minister Han Duck-soo instructed the government to implement "thorough" measures in response to the earthquake, with local reports saying that noise and shaking were also experienced in the cities of Sejong, Daejeon and Cheonan.

Buan resident Yang Seo-yeon told AFP she heard a "thunderous roar" while she was getting ready to go to work in the morning.

"It sounded as if I was in the middle of a huge construction site. Then my house started shaking."

No significant damage had been reported as of Wednesday morning, according to authorities.

Unlike neighbouring Japan, the Korean peninsula rarely experiences significant quakes.

The country's most powerful quake to date was a 5.8-magnitude tremor that struck Gyeongju, also in the southeast, in September 2016.