Top nuclear envoys from South Korea and the United States have agreed to seek the strongest possible sanctions for the DPRK over its latest nuclear test.

US special envoy for the DPRK nuclear issue, Sung Kim, met with his South Korean counterpart, Kim Hong-kyun in Seoul, and they agreed to squeeze Pyongyang further through UN Security Council measures as well as unilateral sanctions.

South Korea's Defence Ministry says it's only a matter of time before the DPRK carries out a sixth nuclear test, adding that Punggye-ri in the north-east was the likely site.