Analysis of recent satellite images of North Korea's nuclear test site shows activity at all three of its tunnel complexes, fuelling speculation of another test ahead of a key political anniversary next week.
North Korea has already conducted two nuclear tests this year -- in January and September -- and experts say it is capable of carrying out a third as soon as the order is given.
Past nuclear tests, and missile launches, have often coincided with special political dates.
The September test took place on the anniversary of North Korea's founding as a state, and the country will celebrate a similar milestone on Monday with the 71st anniversary of the founding of the ruling Workers' Party.
In a posting on its closely-watched 38 North website Friday, the US-Korea Institute at Johns Hopkins University said satellite images dated October 1 showed continuing activity across the Punggye-ri test site in the northeast of the country.
"Activity at the North Portal where the September 9 test occurred, may be for a number of purposes including collecting post-test data, sealing the portal or preparing for another test," the post said.
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