North Korea's surprise nuclear test "should have consequences" for the reclusive state, US Defense Secretary Ashton Carter and his South Korean counterpart agreed in a phone call Wednesday.

"Secretary Carter reaffirmed the ironclad commitment of the United States to the defense of (South Korea,) and that this commitment includes all aspects of the United States' extended deterrence," the Pentagon said in a read-out of Carter's call with South Korean Defense Minister Han Min-koo.

The statement added: "Carter and Han agreed that North Korea's provocations should have consequences," without going into any detail.

Carter visited Seoul and the Demilitarized Zone (DMZ) between the two Koreas in November, where he met with Han and reiterated the United States' longstanding support of South Korea.

About 28,500 US troops are stationed in South Korea and the two forces have very close military ties.

In their call, the two defense chiefs "pledged both sides would coordinate appropriate alliance responses to these provocations."

"Secretary Carter and Minister Han agreed that any such test would be an unacceptable and irresponsible provocation and is both a flagrant violation of international law and a threat to the peace and stability of the Korean peninsula and the entire Asia-Pacific region," the readout stated.