North Korea has boosted biochemical warfare drills as part of its regular winter exercises, intelligence agencies here believe.

"Military intelligence believes that the North has carried out at least a dozen massive biochemical drills since late last year," a senior South Korean official said Thursday.

"We believe that could pose a real threat."

The North has often staged small-scale biochemical warfare drills, chiefly in summer, but not in winter when strong winds could potentially wreak havoc with any hazardous substances that escape into the air.

"These are clearly offensive drills planning a biochemical attack," a military officer here said.

Soldiers frequently practice firing multiple rocket launchers wearing gas masks.

The U.S. Air Force in South Korea has sta rted supplying troops with personal protective equipment as the threat of biochemical attacks from the North mounts

The North is believed to rank third in the world in terms of biochemical warfare capability, with an estimated 2,500 to 5,000 tons of chemical weapons that could kill the entire population of South Korea.

