The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies, however, does not see signs of an imminent test.
It says that the North likely started work on a new tunnel at the northeastern test site at Punggye-ri last May. It estimates that the pile of earth excavated from it has doubled since the start of the year.
The findings are based on commercial satellite photographs, the latest taken Feb. 3. The analysis was provided to The Associated Press ahead of publication Thursday on the institute's website, 38 North.
North Korea has conducted its three previous nuclear explosions at Punggye-ri, the latest in February 2013.
In Seoul on Thursday, U.S. Secretary of State John Kerry and his South Korean counterpart warned North Korea against any possible aggression amid mixed signals from the North over returning to denuclearization talks and improved ties with the South.
The high tension that ensued after last February's test has eased, and North Korea has signaled its willingness to hold dialogue. But the immediate prospects for diplomacy remain uncertain as the North objects to upcoming U.S.-South Korea military exercises, and as the authoritarian government of young leader Kim Jong Un appears to press on with nuclear and missile development.
38 North says that North Korea probably has two other tunnels already complete at Punggye-ri. The entrances of those tunnels are in shadow and not visible in the latest overhead images — underscoring the limitations on what can be gleaned about the North's plans, particularly when the crucial activity takes place underground, out of view. According to the analysis, indications that another test is imminent would become visible between six and eight weeks before an explosion.
This week, South Korea's defense minister, Kim Kwan-jin, told lawmakers in Seoul that North Korea appears ready to conduct its fourth nuclear test, but they have not detected signs that a test is imminent, the Yonhap news agency reported.
"North Korea is clearly keeping all of its options open for the future," said Joel Wit, a former State Department official and editor of 38 North. "There is no evidence to suggest that Pyongyang is preparing for another nuclear test but if a decision were made tomorrow, it could conduct a blast probably by late March or April."
The analysis says the amount of earth heaped near the new tunnel suggests a tunnel of around 1,000 meters in length has been dug into the mountainside, but acknowledges that only a rough estimate can be made from a satellite image. Mining carts can be seen on the tracks to the earth pile, it says.