US and South Korean special forces have been parachuting into North Korea to gather intelligence about underground military installations, a US officer has said in comments carried in US media.

Army Brigadier General Neil Tolley, commander of US special forces in South Korea, told a conference held in Florida last week that Pyongyang had built thousands of tunnels since the Korean war, The Diplomat reported.

"The entire tunnel infrastructure is hidden from our satellites," Tolley said, according to The Diplomat, a current affairs magazine. "So we send (South Korean) soldiers and US soldiers to the North to do special reconnaissance."

"After 50 years, we still don't know much about the capability and full extent" of the underground facilities," he said, in comments reported by the National Defense Industrial Association's magazine on its website.

Tolley said the commandos were sent in with minimal equipment to facilitate their movements and minimize the risk of detection by North Korean forces.

At least four of the tunnels built by Pyongyang go under the Demilitarized Zone separating North and South Korea, Tolley said.

"We don't know how many we don't know about," he admitted.

Among the facilities identified are 20 air fields that are partially underground, and thousands of artillery positions.

In February, South Korea's Yonhap news agency reported that had built at least two new tunnels at a nuclear testing site, likely in preparation for a new test.