North Korea appears to be ready to conduct a nuclear test in as little as two weeks once a political decision is made to move forward, a U.S. think tank said Friday in an assessment based on satellite imagery.

"Satellite photos as recent as December 13 show that Pyongyang is determined to maintain a state of readiness at the area of the Punggye-ri nuclear test site, where a third test is expected even in the dead of winter," the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies said on its website, 38 North.

The institute scrutinized several satellite photos of the North's nuclear test site, taken by GeoEye, a commercial geospatial information provider, in March and April and then again on Oct. 3, Nov. 19, Dec. 2 and Dec. 13.

A previously unidentified structure was spotted that may be intended to protect data-gathering equipment from inclement weather, the institute said.

"The North Koreans have restored the capability to conduct a nuclear test at the south tunnel of the Punggye-ri Nuclear Test Facility in the aftermath of rains this past summer and fall," it added. "They continue to maintain the test site at a state of readiness that could allow them to conduct a detonation as soon as two weeks after such a decision is made."

A possible setback, however, is the continued stream of water that appears to be coming from the tunnel entrance, indicating a possible problem with seepage, which could adversely affect a nuclear device and sensors intended to gather data on and monitor the test, according to the institute.

"Whether this problem is under control or has been solved remains unclear," it said.

 The North carried out two previous tests, in 2006 and 2009, and succeeded in a long-range rocket launch earlier this month.    

The U.S. and its allies are pressing for tougher U.N. sanctions against Pyongyang, but the effort has been apparently hamstrung by China.