A spy plane crashed today during routine training and its two pilots were killed in the second military accident to strike South Korea as it hosted the G20 summit.

South Korea was on heightened alert for possible provocations by communist neighbour North Korea during the two-day G20 summit, but there was no indication of the North's involvement in either the plane accident or the sinking yesterday of a 150-ton naval vessel.

The RF-4C reconnaissance aircraft hit a mountain in Imsil, 185 miles south of Seoul, about 40 minutes after taking off from a base in Suwon, just south of Seoul, an official said.

The pilots' bodies were found at the mountain and no civilians were hurt on the ground, he said.

The air force temporarily grounded all aircraft except planes needed for essential patrol missions and was investigating the cause of the crash, the official said.

South Korea operates about 20 second-hand RF-4C spy planes purchased from the US, the air force said.

The crash came a day after a South Korean navy ship sank after colliding with a larger fishing boat in waters north west of the southern resort island of Jeju.

The G20 summit, which ends today in Seoul, placed a spotlight on security because North Korea has a history of acting provocatively when world attention is focused on South Korea.

The Korean peninsula officially remains in a state of war because the 1950-53 Korean War ended in an armistice, not a peace treaty.