A senior South Korean presidential official says North Korea has been testing a nuclear detonation device apparently in preparation for a nuclear test.

Kim Tae-hyo, first deputy director of the presidential National Security Office, told reporters on Wednesday the possibility of a nuclear test in the next day or two is low, but there is certainly a possibility after that.

Kim revealed that operational tests of a nuclear triggering device have been detected at a location away from nuclear test site Punggye-ri, and the tests have been carried out multiple times over the past several weeks.

He added that North Korean authorities are near the final preparation stage for a nuclear test of a scale and quality they want.

Last week, South Korea's National Intelligence Service told a parliamentary committee the North has completed preparations for its seventh nuclear test, and is now gauging its timing.