South Korea's military is warning that North Korea may be preparing to conduct another underground nuclear test.
Defense Ministry spokesman Kim Min-seok said the assessment is based on increased activity at the North's Punggye-ri nuclear test site.
At a Tuesday press briefing, Kim declined to comment on the specific nature of the activity, but warned it could be a decoy by the North.
"Our military bear it in mind that North Korea is able to conduct a sudden nuclear test in a short period of time or it may 'disguise' us by pretending to have a nuclear test as we can see from the past cases."
He said Seoul, along with Washington, is closely monitoring the site and has formed a special task force in response.
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 this week's visit to Seoul by U.S. President Barack 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 command to conduct a nuclear test at any time.