As part of this effort, United States Central Intelligence Agency (CIA) personnel operating from a base at Udon Thani, Thailand, took over the support of between 30,000 and 36,000 irregulars, including Hmong guerrillas who bore the brunt of the fighting in northern Laos. A CIA-chartered airline, Air America, dropped rice and ammunition from its C-46s and C-47s to isolated Hmong outposts, which were sometimes behind enemy lines. A variety of short takeoff and landing aircraft used dirt airstrips carved out of the jungle by the Hmong. The irregulars, who became known as the Secret Army, were instrumental in helping to rescue a large number of United States airmen who were shot down over Laos. By this time, Hmong leader Vang Pao had risen to the rank of general in the Royal Lao Army and commanded the Second Military Region.