North Korea may conduct an underground nuclear warhead test when it detonates another atomic device, the defense ministry's spokesman said Monday.

The remarks by spokesman Moon Sang-gyun were in response to a media inquiry on whether North Korea's fifth nuclear test would involve a nuclear warhead.

A nuclear warhead test could involve either the underground detonation of a missile-capable nuclear device or a warhead detonation without the use of nuclear materials, Moon said in a regular press briefing.

"Given the latest development, North Korea has entertained the possibility of carrying out an underground nuclear (warhead) test and we are keeping close tabs on it," he noted.

North Korean leader Kim Jong-un ordered a nuclear warhead detonation test "in the earliest possible time" last month, and the South Korean military is paying close attention to relevant movements, he also said.

North Korea's four previous nuclear tests, including the latest in January, were all carried out underground at Punggye-ri in the northeastern part of the country.

Even if North Korea conducts a nuclear warhead test, it would be hard for South Korea to tell whether it was a conventional nuclear test or a new warhead test, Moon also noted.

South Korean officials have said a recent sharp increase in vehicle and human movements at the North's Punggye-ri nuclear test site may point towards an imminent nuclear detonation.

The possible nuclear test may come before North Korea's much-trumpeted ruling party congress in early May where leader Kim will attempt to flaunt his economic and political achievements.

The possibility of an imminent nuclear test has gained more traction since North Korea botched the first test of its intermediate-range Musudan ballistic missile last week.

Officials said Pyongyang may carry out another provocation to make up for the missile failure. (Yonhap)