Tremors were felt around the country, including in Seoul and Busan. The residents of Pohang were told to evacuate and reports emerged about buildings shaking following the tremors.
According to local TV footage, some of the buildings saw external wall damaged and window broken.
Korea Hydro & Nuclear Power (KHNP) said the six nuclear reactors in Gyeongju, about 45 kilometers south of the epicenter, as well as other reactors on the southeastern coast are all functioning normally.
The nuclear power plant Singori 3 in Ulsan is believed to remain unaffected by the quake.
South Korean President Moon Jae-In, on his way to Seoul after attending the Asean summit in Manila, is set to hold an emergency meeting upon arrival, his office said.