North Korea has hinted that a nuclear test is imminent, saying leader 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.

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 North Korea's Central Military Commission, attended by heads of the army, in which he spoke about "bringing about a great turn in bolstering military capability".

The official Korean Central News Agency (KCNA) said Mr Kim spoke to his generals about "defending the security and sovereignty of the country", and said participants vowed to "thoroughly implement the military tasks set forth".

North Korea had earlier threatened to carry out the test in response to tightened UN sanctions imposed for a long-range rocket launch last December.

Over the past week Pyongyang has issued a series of daily warnings threatening action over the sanctions, including a promise on Saturday of the "toughest retaliation".

Several observers in South Korea believe the nuclear test will come before Lunar New Year starts on February 10.

ABC/wires