Writing on their blog, 38 North, the team from Johns Hopkins University said commercial satellite imagery taken of the Punggye-ri nuclear test site in North Korea suggests "significant" acceleration of activity there.
"Once a decision is made in Pyongyang, indicators visible in satellite imagery of an impending nuclear test can appear 4-6 weeks prior to the test, both near the tunnel entrance and in other areas of the site," the research team said Thursday.
They cautioned, however, that it couldn't determine the cause of the activity, stating "there are no signs that a test is in preparation."
Last week, Johns Hopkins University said satellite imagery suggested Pyongyang is working to enhance its rocket-launch capabilities at its Sohae Satellite Launching Station.
China last year said it was time to bring negotiating partners back to the table to ensure security on the Korean Peninsula.
Six-party talks between the two Koreas, China, the United States, Russia and Japan stalled in 2009. North Korea has conducted two nuclear tests since then.