Relevant authorities were still verifying the facts, the statement said.
The JCS said troops were sent but could not locate the person crossing the military demarcation line into North Korea around 10:40 PM.
A military official said security cameras had previously captured the person climbing a barbed wire fence around 6:40 p.m. Saturday, but the CCTV security guard had missed it.
The official said they were currently in the process of identifying the person and it is unknown if the person was still alive.
“We sent a message to North Korea yesterday to protect our people and there has been no response from North Korea,” said Boo Seung-chan, a spokesman for South Korea’s defense ministry.
There were “no unusual moves by the North Korean military” regarding Sunday’s incident, he added.
North Korean state media has no coverage of the crossing of the DMZ overnight.