The head of the International Atomic Energy Agency(IAEA) has expressed concern over the continual detection of "deeply troubling" signs of activity at North Korea's Punggye-ri nuclear testing facility.

At the United Nations agency's board of governors meeting in Vienna, Austria, on Monday, Director-General Rafael Grossi said the Punggye-ri facility remains prepared to support a nuclear test.

He said indications of activity continue to be observed near Tunnel Number Three entrance at the facility, while the road to the former Tunnel Number Four has been reconstructed.

Grossi also said the regime appears to be operating the five-megawatt reactor as well as what is believed to be a centrifuge enrichment facility at its Yongbyon nuclear facility.

He added that signs of possible tests of the light water reactor cooling system were detected between late September and early October, while indicators of changes being made to the reactor's cooling water outlet channel were spotted in October.