South Korean officials say there's increased activity at a nuclear test site in North Korea.
This comes just days ahead of President Obama's arrival in South Korea on his four-nation trip to Asia.
The South thinks North Korea may either be planning to stage a surprise nuclear test or a pretend one.
The communist North's last nuclear test in early 2013 led to greater tensions on the Korean peninsula and further United Nations sanctions against Pyongyang.
Multinational talks aimed at bringing the North's nuclear activities to a halt have been stalled for years.