North Korea has hinted that a nuclear test is imminent, saying Kim Jong-un has made an important speech to the country's military commanders.
Recent satellite images have revealed intense activity at North Korea's nuclear test site and analysts believe Pyongyang has completed preparations for its third nuclear test.
The ABC reports engineers have now covered the entrance to a tunnel that is believed to lead to the underground test site at Punggye-ri, in the north-east of the country.
State media has confirmed that Mr Kim chaired a meeting of the Central Military Commission.
Several observers in South Korea believe the nuclear test will come before the Lunar New Year starts on 10 February.