The United States is "closely" monitoring possible provocation of Democratic People's Republic of Korea (DPRK), the White House said Tuesday, as U.S. President Barack Obama embarked on a four-country tour of Asia.

"North Korea has a history of taking provocative actions, and we are always mindful of the possibility that such an action could be taken," White House spokesman Jay Carney told reporters abroad Air Force One as Obama left for a week-long Asian trip, which will take him to Japan, South Korea, Malaysia and the Philippines.

South Korea's Defense Ministry on Tuesday warned of a "surprise " nuclear test of the DPRK, citing increased activities in its main underground test site.

Carney said the United States has seen the reported activities of the DPRK, noting that "there is a kind of cyclical nature to the provocative actions that North Korea tends to take."

But the spokesman declined to discuss the information the United States has about DPRK's activities or speculate about its possible actions.

Obama is scheduled to arrive in South Korea on Friday for a two- day trip after wrapping up a state visit to Japan.

Pyongyang has conducted three nuclear tests since 2006, with the latest one in February last year.