The United States is closely monitoring the situation on the Korean peninsula, after reports that North Korea may be planning another nuclear test ahead of President Barack Obama's trip to the peninsula.
State Department spokeswoman Jen Psaki said the U.S. continues to urge North Korea "to refrain from actions that threaten regional peace and security.''
Washington was responding to a warning by the South Korean military that the North may be preparing another underground nuclear test.
Defense Ministry spokesman Kim Min-seok said that assessment is based on increased activity at the North's Punggye-ri nuclear test site.
Mr. Obama travels to Japan Wednesday and to Seoul Friday. He is expected to discuss ways to deal with North Korea's nuclear weapons program.
The North conducted nuclear tests in 2006, 2009 and 2013 in violation of United Nations Security Council resolutions.
Last month, Pyongyang warned it was preparing to stage an unspecified "new form" of nuclear test. It is believed to be working on, but still far from perfecting, a nuclear warhead small enough to fit on an intercontinental ballistic missile.
Some analysts have predicted North Korea could stage a provocation during the visit to Seoul by President Obama.
The behavior of North Korea's secretive, communist leadership is famously difficult to predict.
South Korean leaders have warned in recent months that Pyongyang could give the order to conduct a nuclear test at any time.