North Korea is capable of detonating another nuclear device at any time at the country's main test site, South Korea has said.

The North launched its most powerful nuclear test on Friday, sparking concerns that the country is making headway in its ambitions to develop sophisticated warheads for missiles.

South Korea's Defence Ministry spokesman Moon Sang Gyun told reporters that South Korean and US intelligence officials believe the North has the ability to detonate another atomic device any time, but declined to give further details.

Shortly after the North's test, South Korea's leading military intelligence officer Kim Hwang Rok said North Korea has two or three unused tunnels at the Punggy-ri test site from where it can conduct an additional test.

Citing unidentified government sources, South Korea's Yonhap news agency reported signs that the North had finished test preparations in one previously unused tunnel at Punggy-ri.

Seoul, Washington and their allies have pledged to apply more pressure and sanctions on Pyongyang after its latest nuclear test.