"North Korea seems to be fully prepared to carry out a nuclear test at any time when the order is given by its leader (Kim Jong-un)," said Seoul's Unification Ministry spokesman Jeong Joon-hee.
"South Korea is closely monitoring every move at the North's test site in close cooperation with the US," Yonhap news agency quoted Jeong as saying.
Seoul based the warning on its own intelligence inputs and those from Washington that indicated increased vehicular and personnel movement at the Punggye-ri nuclear site in northwestern North Korea.
Images taken on July 7 and released by specialised portal 38 North, also revealed intense activity at the site, where the North Korean army had carried out its last two nuclear tests, EFE news reported.
On January 6, Pyongyang had detonated underground what it said was a hydrogen bomb, although subsequent analysis showed it was a less powerful device.
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