During the entire period of 1970 – 74 , FRELIMO intensified guerrilla operations , specialising in urban terrorism . The use of landmines also intensified , with sources stating that they had become responsible for two out of every three Portuguese casualties . During the conflict , FRELIMO used a variety of anti @-@ tank and anti @-@ personnel mines , including the PMN ( Black Widow ) , TM @-@ 46 , and POMZ . Even amphibious mines was used , such as the PDM . Mine psychosis , an acute fear of landmines , was rampant in the Portuguese forces . This fear , coupled with the frustration of taking casualties without ever seeing the enemy forces , damaged morale and significantly hampered progress .