U.S. Monitoring North Korea for Potential Nuclear Test

North Korean leader Kim Jong Un North Korea's Korean Central News Agency

WASHINGTON (Reuters) - The United States said on Tuesday it was closely monitoring the situation on the Korean peninsula after reports that North Korea may be planning another nuclear test and urged Pyongyang not to take any step that would threaten regional peace.

"We have certainly seen the press reports ... regarding possible increased activity in North Korea's nuclear test site," U.S. 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 Korean news reports earlier on Tuesday quoted the South Korean government as saying 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.

"A lot of activity is currently being seen, 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," South Korea's Yonhap news agency quoted South Korean Defense Ministry spokesman Kim Min-seok as saying.

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.

North Korea's deputy ambassador to the United Nations said on April 4 the world would have to "wait and see" when asked what was meant by a "new form" of test.

Nuclear expert Jeffrey Lewis, of the Monterey Institute of International Studies in the United States, said this month that North Korea's reference to a new form of test could mean simultaneous detonation of two or more devices as part of a program of more intense nuclear testing expected over the next few years.

While North Korea has detonated several nuclear devices, analysts have expressed doubt it currently has the technical capability to reliably mount a nuclear warhead on a missile.

The United States said it held "productive" talks with China on North Korea last week, part of stepped up international diplomacy after Pyongyang's warning.

China has warned against any action that could lead to the escalation of tensions on the Korean Peninsula.

Diplomats have said it is possible the Security Council will respond to the North Korean missile tests by expand a U.N. blacklist to include additional North Korean entities involved in Pyongyang's missile program. But they said it could take weeks to reach agreement.

The council expanded its sanctions on North Korea after Pyongyang's February 2013 atomic test, its third nuclear detonation since 2006.