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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.
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."
North Korea also continues to move equipment and personnel that could be used to launch ballistic missiles, the US officials said. They caution, however, it's impossible for the United States to know when a mobile launch or an underground nuclear test will happen. North Korea has a history of moving military personnel and gear around to deceive satellites the regime knows are watching.
The United States is maintaining the presence of a WC-135 aircraft in the region that can conduct air sampling after a suspected underground test. In past instances of Pyongyang's nuclear testing, one of the first signs that an underground explosion has happened is seismic activity picked up by the US Geological Survey in the location of the North Korean test site.
All of this comes as the Trump administration is reviewing whether there are fresh options for dealing with North Korea, a situation that the Trump and Obama administrations have agreed is extremely difficult to manage.
Privately, US military commanders have said any preemptive strikes by the US would likely result in a North Korean attack on Seoul, leading to disastrous consequences.
On an overseas trip to Asia earlier this month, Secretary of State Rex Tillerson acknowledged in Tokyo, "I think it's important to recognize that the diplomatic and other efforts of the past 20 years to bring North Korea to a point of denuclearization have failed."
That sentiment was already expressed late last year by then-Director of National Intelligence James Clapper, who called the effort to get North Korea to give up its nuclear program "probably a lost cause."