South Korea was clearing up Tuesday after being struck by its most powerful earthquake since records began.

The 5.4 magnitude quake late Monday sent people scurrying from buildings, unused to the kind of seismic events that regularly shake neighbouring Japan.

Seismologists said the quake was a natural geological phenomenon, and unrelated to the "artificial earthquake" caused last week when North Korea carried out an underground nuclear test.

There were no reported deaths, but six people suffered minor injuries after being hit by falling objects, the Ministry of Public Safety and Security reported.

South Korea's lively social media was flooded with images of shattered storefront windows and people fleeing apartments in panic, some with children in their arms.

Some spent the night in shelters or in their cars.

Kakaotalk, the country's largest mobile app, which is used by 40 million people, partially went down, the company said, as users rushed to exchange messages in the aftermath of the quake.

Television footage showed bottles falling from shelves at a store, and long cracks that had developed in a basement parking lot.

Reactors at four nuclear power plants were shut down for safety inspections, and were expected to remain off line for several days, but there were no indications of any damage.

Korea Hydro & Nuclear Power Co., which operates the country's 24 nuclear plants, it had halted operations at four sites "on the off-chance" they might have suffered damage.

"All our plants were designed to sustain quakes of up to 6.5 magnitude", a company spokesman told AFP.

The US Geological Survey said the country had been hit by two quakes, the first of which had a magnitude of 4.9. It was followed an hour later by the more powerful 5.4 magnitude shake.

Korean seismologists, who put the magnitude of the second quake at 5.8, said it was the most powerful to hit since records began in 1978.