A U.S. research group says there is "little evidence" North Korea will conduct a nuclear test during President Barack Obama's upcoming visit to Seoul.
South Korea said this week it detected increased activity at the North's Punggye-ri nuclear test site, suggesting an underground test could be imminent.
Some North Korea watchers expect a provocation during Obama's visit, which starts Friday. They note that the North recently warned it could conduct another nuclear test.
The U.S.-Korea Institute confirmed Wednesday that commercial satellite imagery suggests the North has, indeed, begun new operations at Punggye-ri.
However, the new activity appears to consist of moving crates, boxes and materials, and has not reached the level of intensity seen before past tests.
The U.S.-Korea Institute is affiliated with the Johns Hopkins School of Advanced International Studies. Its North Korea reports, posted on its 38 North website, are closely watched.
The institute said the activities at Punggye-ri could represent an early stage of preparations for a nuclear test, or could simply show that North Korea is conducting routine maintenance.
The U.S. State Department said Tuesday it is closely monitoring the situation on the Korean peninsula. It urged Pyongyang "to refrain from actions that threaten regional peace and security.''
The developments come ahead of President Obama's visit to Seoul Friday -- part of his eight-day, four-nation tour of Asia. A major focus of the trip will be North Korea's nuclear program.
The North conducted nuclear tests in 2006, 2009 and 2013 in violation of United Nations Security Council resolutions.
Last month, Pyongyang warned it was preparing to stage an unspecified "new form" of nuclear test. It is believed to be working on, but still far from perfecting, a nuclear warhead small enough to fit on an intercontinental ballistic missile.