As a part of the KPA build-up in the south, the untried KPA 7th Division also arrived near Masan with another 10,000 men. The 7th Division occupied key ports to protect the 6th Division against possible amphibious landings in its rear. Eventually, though, the division was committed to combat in conjunction with other KPA units. The simultaneous attacks were hoped to overwhelm UN lines. On August 17, the KPA resumed their attack. A battalion of KPA troops drove the South Korean National Police out of T'ongyong but did not hold it long. UN naval forces heavily shelled T'ongyong as three companies of South Korean Marines from Koje Island made an amphibious landing near the town. The ROK force then attacked the KPA and, supported by naval gun fire, drove them out. The KPA at T'ongyong lost about 350 men, the survivors withdrew to Chinju. The reinforced KPA had advanced on the 25th Infantry Division defensive line and had begun a series of probing attacks that were to continue throughout the month, sometimes of battalion strength. Most of these attacks came in the high mountains west of Haman, in the Battle Mountain, P'il-bong, and Sobuk-san area. There the 6th Division attacked any UN-held terrain features that afforded observation of its supply and concentration area in the deeply cut valley to the west. The KPA 6th Division shifted its axis of attack and its main attacking effort to the northern part of the Chinju-Masan corridor just below the Nam River in the 35th Infantry's sector. The 35th Infantry set to work to cover its front with trip flares, but they were in short supply and gradually it became impossible to replace them. Illuminating flares were also in short supply, and the reserve stocks had deteriorated to such a degree that only about 20 percent of the supply issued to the regiment was effective. Even when employed, the time lapse between a request for them and delivery by the big howitzers allowed some KPA infiltration before the threatened area was illuminated. The 64th Field Artillery Battalion, with C Battery, 90th Field Artillery Battalion, attached, and A Company, 88th Medium Tank Battalion, supported the regiment. Three M4A3 Sherman tanks, from positions at Komam-ni, acted as artillery and placed interdiction fire on Chungam-ni. Six other M26 Pershing tanks in a similar manner placed interdiction fire on Uiryong across the Nam River.