Story highlights South Korea says a test can happen as soon as North Korean leader Kim Jong Un orders it

US maintains presence of WC-135 Combat Shadow aircraft in region to conduct air sampling after possible test

Washington (CNN) Specific intelligence indicators now show that North Korea is ready for its sixth underground nuclear test at any time, two US officials told CNN on Friday.

Recent satellite imagery indicates a potentially significant change at the Punggye-ri test site, the officials said. For weeks, satellites had observed extensive activity on the surface, including vehicles, personnel and equipment, as well as two tunnel entrances being dug out.

But the most recent imagery shows that activity has now stopped. That is a similar change in the pattern of activity just before previous tests, indicating all final preparations are now complete, officials said.

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The South Korean government issued a statement Friday 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 a press briefing. 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."

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