The United States said Tuesday it was watching "very closely" following South Korean warnings that North Korea may be preparing a fourth nuclear test ahead of a visit to Seoul by President Barack Obama.

"North Korea has a history of taking provocative action and we are always mindful of the possibility that such an action could be taken," White House spokesman Jay Carney told reporters aboard Air Force One as Obama departed for his week-long trip to Asia.

Carney said any action by North Korea would "most likely be in violation of numerous commitments that the DPRK (North Korea) is bound by, but of course that is something that they unfortunately have done many times."

He declined to comment on the validity of a South Korean defense ministry briefing which cited increased activity at North Korea's main nuclear test site.

"We'll be watching it very closely," Carney said.

The South Korean claims were backed up to some degree by the US-Korea Institute at Johns Hopkins School of Advanced International Studies, which said that recent commercial satellite images showed "new operations" at the North's Punggye-ri test site.

Imagery from March to as recently as Saturday show the movement of crates, boxes and possibly wood near the entrances, according to analysis posted on the institute's website, 38 North.

"It is possible these materials are being moved inside those tunnels," the institute said.

But it qualified the discoveries by saying the activity "falls short" of what was observed there in the weeks prior to past detonations.

It "could represent an early stage of preparations for a test or may be intended for a less provocative purpose, such as conducting maintenance after a long winter."

Obama is due to reach Seoul Friday during an Asia swing which also includes stops in Japan, Malaysia and the Philippines.

North Korea on Monday slammed Obama's Asian tour as "reactionary and dangerous," saying it would only serve to "escalate confrontation and bring dark clouds of a nuclear arms race" over the Korean peninsula.

North Korea has carried out three nuclear tests in the past eight years -- in October 2006, May 2009 and February 2013.