Two pilots died on Friday when a South Korean spy plane crashed into a mountain during a training flight over the South's territory, military officials said.
The RF-4C reconnaissance plane came down during a flight above Imsil county neat Jeonju city 260 kilometres (160 miles) south of Seoul, a defence ministry spokesman said.
The plane was on a low-altitude training flight and the cause of the accident was not yet known, he said.
"Wreckage was found on a mountain and a rescue operation is under way to retrieve the bodies of two airmen," an air force spokesman told AFP.
The plane is normally used for photographic reconnaissance along the tense border with North Korea.
The photographic reconnaissance verson of the F-4 Phantom fighter jet was produced by US aircraft maker McDonnell Douglas in 1966 and introduced into the South's air force in the late 1980s.
Friday's crash was the second involving the planes. In 2008 the two pilots ejected safely before impact.
The South's military has been on top alert during a G20 summit of world leaders in Seoul, which ends on Friday.