South Korean media report that work at North Korea's nuclear test site has entered its final stage, heightening speculation that a third atomic test is imminent.
The reports, which come from government sources in South Korean, suggest activity at the nuclear site has increased.
An ABC correspondent said it is believed the assessment is based on satellite images of the area, although any nuclear device will be tested underground.
There are other reports that a tunnel leading to the underground test site has been sealed and all that is needed is approval to fire from Pyongyang.
North Korea has threatened to detonate a nuclear device in retaliation for new sanctions passed by the UN Security Council.
Two previous nuclear tests were held in 2006 and 2009.
A long-range rocket carrying a satellite into space was launched on 12 December.
State media in China indicated on 25 January that Beijing would decrease aid to Pyongyang if it goes ahead with a nuclear test.
"If North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance," said the Global Times in an editorial.