The United States said on Tuesday it was watching the Korean peninsula closely after reports that North Korea may be planning another nuclear test and it urged Pyongyang not to take any step that would threaten regional peace.
South Korean news reports quoted the South Korean government as saying on Tuesday that heightened activity had been detected at North Korea's underground nuclear test site, indicating possible preparations for another atomic test.
The reports come just before U.S. President Barack Obama is due in Japan and South Korea, where he will discuss ways to deal with North Korea's nuclear weapons program. Obama is due in Tokyo on Wednesday and in Seoul on Friday.
"We have certainly seen the press reports ... regarding possible increased activity in North Korea's nuclear test site," State Department spokeswoman Jen Psaki said. "We are closely monitoring the situation on the Korean peninsula."
"We continue to urge North Korea to refrain from actions that threaten regional peace and security and to comply with its international obligations and commitments," she told a regular news briefing.
South Korea's Yonhap news agency quoted South Korean Defense Ministry spokesman Kim Min-seok as saying that "a lot of activity" was being seen at the Punggye-ri test site.
"So our forces are keeping in mind the possibility that North Korea may suddenly conduct a nuclear test in a short period of time, or as in previous cases, deceive us with what appears to be a nuclear test.
"North Korea warned last month it would not rule out a "a new form" of nuclear test after the United Nations Security Council condemned Pyongyang for launching ballistic missiles into the sea.
On the Air Force One flight carrying Obama to Asia, White House spokesman Jay Carney was asked about the reports that North Korea may be preparing a nuclear test.
"North Korea has a history of taking provocative actions and we are always mindful of the possibility that such an action could be taken,' he told reporters.