<P> Civilian labor was crucial to VC / NVA success, and was deployed in building fortifications, transporting supplies and equipment, prepositioning material in readiness for an operation, and general construction such as road repair . Labor was recruited primarily by impressment / draft, or as a way to pay off VC taxes, although volunteers motivated by ideology also took part . Twelve to sixteen hours of work per day were expected of laborers . Civilians undertook various pledges as directed by the regime (the "three readies", the "three responsibilities" among others,) as part of a high mobilization of the population for total war in the North and areas controlled by the VC / NVA in the South . </P> <P> Load bearing by porters was greatly enhanced by the use of ingenious "steel horses"--bicycles specially modified by widening the handlebars, strengthening the suspensions and adding cargo pallets . Guided by two men, the specially modified bikes could move 300--400 pounds, several times that of a single porter . Older men made up many of the long - term laborers as those younger were drawn off into combat and female labor was used extensively in a wide range of logistics tasks . </P> <P> Communist forces also made extensive use of Vietnam's large network of rivers and waterways, drawing heavily on the Cambodian port of Sihanoukville, which was also off - limits to American attack . Some 80% of the non-food supplies used by the VC / NVA in the southern half of South Vietnam moved through Sihanoukville . </P> <P> Some port areas of North Vietnam were also vital to the logistical effort, as were ships by socialist nations that fed the continual stream of war material . Attacks on these were also forbidden by American policymakers . Until late in the War, American pilots, hindered by their government's rules of engagement, could only watch helplessly as munitions, heavy weapons and advanced components like SAM missile batteries were unloaded at such harbors as Haiphong . By 1966, some 130 SAM batteries were in North Vietnam by US estimates, manned primarily by Russian crews . </P>

Over which of these did the north vietnamese primarily supply their troops fighting in the south