SEOUL — 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 that the North could be moving towards what would be its fourth nuclear test prompted the U.S. State Department to urge Pyongyang to "refrain from actions that threaten regional peace".

North Korea continually works on improving testing at its Punggye-ri site. Satellite imagery analyzed by 38North, which is part of Johns Hopkins University, said that while there had been a pickup in activity, there were few signs of an imminent test.

"Recent operations at Punggye-ri have not reached the high level of intensity - in terms of vehicle, personnel and equipment movement - that occurred in the weeks prior to past detonations," it said.

"Moreover, other possible indicators present before the North Korean nuclear tests in 2009 and 2013, such as communications vans and a satellite dish intended to transmit pre-test data, have not been spotted."

Work at the site is seasonal and often picks up in the Korean spring. Some earlier satellite images had suggested the North was digging multiple tunnels, which could possibly indicate that it was planning more than one nuclear detonation.

38North acknowledges that the commercial satellite imagery it uses may present an incomplete picture of the highly secretive state.

The comments by the South Koreans have been repeated frequently in the past and there was little new in their assessment this week that the North could be ready to launch a new test at short notice.

North Korea has been heavily sanctioned for its nuclear and long-range rocket programs but has pressed ahead with both. Military experts say the North would lose any conventional war with South Korea and the United States and is seeking to develop nuclear weapons as a deterrent.

It has also used the nuclear program to attempt to wring concessions and aid from the United States. Washington has said that it would be open to talks if North Korea abandons its nuclear program.

North Korea describes its nuclear weapons as a "treasured sword" that it will never abandon.