A 5.4-magnitude earthquake shook South Korea's southeastern coast on Wednesday afternoon, but no casualties were immediately reported.

People evacuated buildings near the epicentre and the shaking was felt in Seoul.

The quake was centred off the coast about 9.3km northwest of the port city of Pohang, according to the US Geological Survey.

South Korea's state-run Korea Meteorological Administration measured the quake at the same strength but said the epicentre was inside Pohang.

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Residents in the capital, more than 300km away, felt tremors and said their buildings shook.

No injuries or major damages were immediately reported, according to the Korea Meteorological Administration.

It's the strongest quake in South Korea since a 5.8-magnitude occurred near the ancient city of Gyeongju, which is close to Pohang, in September 2016, Korea Meteorological Administration officials said.

South Korea has relatively little seismic activity, unlike neighbouring Japan.