South Korea is warning that the communist North could follow a planned rocket launch in the next week with another nuclear test.
Seoul has released a report alleging that work is again underway at the site of the North's previous nuclear detonations in the country's north-east.
The report says recent satellite images show large piles of dirt at the entrance to a tunnel and suggests the dirt could be used to plug the tunnel before the North conducts a nuclear test.
South Korea and Japan are already on high alert for the launch which they say is a disguised ballistic missile test.
The North Korean Government says the rocket is carrying a satellite.