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

"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 left for a week-long Asia trip, which would take him to Japan, South Korea, Malaysia and the Philippines.

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

Carney said the United States has seen press reports of the alleged activities of Pyongyang, but he declined to discuss the information the United States has about DPRK's activities or speculate about them.

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

The DPRK has repeatedly urged the United States to stop hostile policies toward Pyongyang, and vowed to continue to develop military capabilities to deter any US threats.

The country also blamed Washington for creating tension on the Korean Peninsula, saying the United States has been trying to strangle it by using political, military and economic means.

The DPRK fired two Rodong medium-range ballistic missiles on March 26, which prompted the UN Security Council to hold an extraordinary session to censure its violation of UN resolutions that ban Pyongyang from testing ballistic missile technology.

In response, on March 30, the DPRK's Foreign Ministry issued a warning that it would "not rule out a new form of nuclear test" to enhance its nuclear deterrence, accusing the United States of "acting rashly" at the UN Security Council.

In February, the DPRK's Defense Minister Kim Kwan-jin said preparations for another nuclear test were already completed, noting that nuclear tests and long-range missile launches could be carried out at any time upon the decision of the DPRK leadership.

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

Amid the recent tension, China on Tuesday again called on all parties concerned to remain calm, exercise restraint and adhere to the goal of realizing a nuclear-free Korean Peninsula.