"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."
"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," defence ministry spokesman Kim Min-Seok told reporters earlier.
"We'll be watching it very closely," Carney said.
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Obama is due to reach Seoul on Friday during a swing through Asia which also includes stops in Japan, Malaysia and the Philippines.
North Korea slammed Obama's Asian tour as "reactionary and dangerous" yesterday, saying it would only serve to "escalate confrontation and bring dark clouds of a nuclear arms race" over the Korean peninsula.