The United States has said that North Korea is ready for its sixth underground nuclear test at any time.
US officials pointed out that their observation is based on specific intelligence indicators.
Satellite imagery indicates a potentially significant change at the Punggye-ri test site, the officials told CNN on Friday.
For weeks, the satellites had observed extensive activity on the surface, including vehicles, personnel and equipment, as well as two tunnel entrances being dug out.
But the latest imagery shows that activity has now stopped, "a similar change in the pattern also observed before previous tests, indicating all final preparations were now complete."
The South Korean government issued a statement on Friday also saying a test can happen as soon as North Korean leader Kim Jong-un orders it.
"It is assessed that North Korea is ready to carry out a nuclear test anytime if its leadership decides to do so," Lee Duk-haeng, spokesman for South Korea's the Ministry of Unification, said in the statement.
He said that "South Korean and US intelligence authorities evaluate that North Korea is ready to carry out a nuclear test anytime on the leadership's decision."
North Korea also continues to move equipment and personnel that could be used to launch ballistic missiles, the US officials told CNN.
They warned that it was impossible for the US to know when a mobile launch or an underground nuclear test will happen.
According to the officials, the US is maintaining the presence of a WC-135 aircraft in the region that can conduct air sampling after a suspected underground test.
(With IANS inputs)