North Korea is unlikely to be ready to stage a nuclear test timed to coincide with U.S. President Barack Obama's visit to Asia, a respected think tank said on Wednesday, based on its assessment of satellite imagery.

Comments by South Korea's Foreign Ministry on Tuesday that the North could be moving toward what would be its fourth nuclear test prompted the U.S. State Department to urge Pyongyang to "refrain from actions that threaten regional peace."

Recent commercial satellite imagery indicates North Korea has begun new operations at Punggye-ri, said 38 North, a North Korea monitoring website run by Johns Hopkins University's School of Advanced International Studies in Washington.