US and South Korean special forces have been parachuting into North Korea to gather intelligence on Pyongyang’s tunnels, which are used as air fields and artillery positions that are hidden from satellites, according to current affairs magazine, The Diplomat.

Army Brigadier General Neil Tolley, commander of US special forces in South Korea, said that some tunnels had existed since the Korean war, with at least four going under the Demilitarized Zone separating the north from the south. “After 50 years, we still don’t know much about the capability and full extent of the underground facilities”, he commented on the National Defense Industrial Association’s magazine.

US and South Korean soldiers were sent to do the special reconnaissance with minimal equipment to minimize the risk of detection by North Korean forces. Yonhap news agency of South Korea reported that two tunnels had been built at a nuclear testing site, raising speculations of a third nuclear test.

South Korea requested to purchase missiles and helicopters from the US.