SEOUL (AFP, REUTERS) - An undersea earthquake off the coast of North Korea was not caused by a nuclear test, South Korean media said on Thursday (July 13).

The 5.9-magnitude quake struck about 190km south-east of the reclusive state's third largest city, Chongjin, in the early hours of Thursday, according to the United States Geological Survey (USGS).

The quake was very deep, at 538km below the seabed, and was unlikely to have caused any damage.

An earthquake of that size is unusual for that area but not unprecedented, USGS seismologist Julie Dutton told Reuters. She said the last large quake in that part of the East Sea/Sea of Japan was in 1994.

North Korea causes seismic events when it conducts underground nuclear bomb tests, but Ms Dutton said there was nothing to indicate this quake was a manmade event.

North Korea has staged five nuclear tests - including two last year - and has made a significant progress in its missile capability under leader Kim Jong Un, who took power in 2011.

USGS geophysicist John Bellini also confirmed that the latest quake, which did not trigger a tsunami warning, was not caused by a nuclear test.

"It occurred at 500km below the seabed," he told Yonhap news agency. "It's a natural earthquake."

A Pentagon spokesman also said initial indications showed that the quake did not result from a nuclear test.

Tensions between North Korea and the US soared earlier this month when the North test-fired a missile - which Pentagon said was an intercontinental ballistic missile (ICBM) - for the first time. It was an apparent game-changer in its confrontation with Washington over its nuclear and missile programmes.