 With the ROK army in retreat , the 766th Regiment , 549th Unit , and KPA 's 5th Divisions all advanced steadily south along the eastern roads without encountering much resistance . Across the entire front the North Korean Army had successfully routed the South Koreans and was pushing them south . The 766th Regiment acted as an advance force , attempting to infiltrate further inland as it moved through the mountainous eastern region of the country . The rugged terrain of the eastern regions of Korea , poor communication equipment , and unreliable resupply lines thwarted the South Korean resistance . The North Koreans used this to their advantage in advancing but they began to experience the same problems themselves . The 5th Division and the two other units began advancing south slowly and cautiously , sending strong reconnaissance parties into the mountains to ensure they would not be threatened from the rear . However , this more cautious advance began to give the South Koreans valuable time to build up further south . By June 28 , the 766th had infiltrated into <unk> @-@ <unk> from Uljin and was moving toward <unk> , <unk> and <unk> in order to block communications between <unk> and <unk> , where United States Army forces were landing in an attempt to support the collapsing ROK Army . 
