LASHKAR GAH, Afghanistan — Farmers from the district of Marja, which since February has been the focus of the largest American-led military operation in Afghanistan, are fleeing the area, saying that the Taliban are terrorizing the population and that American troops cannot protect the civilians.

The departure of the farmers is one of the most telling indications that Taliban fighters have found a way to resume their insurgency, three months after thousands of troops invaded this Taliban stronghold in the opening foray of a campaign to take control of southern Afghanistan. Militants have been infiltrating back into the area and the prospect of months of more fighting is undermining public morale, residents and officials said.

As the coalition prepares for the next major offensive in the southern city of Kandahar, the uneasy standoff in Marja, where neither the American Marines nor the Taliban have gained the upper hand and clashes occur daily, provides a stark lesson in the challenges of eliminating a patient and deeply rooted insurgency.

Over 150 families have fled Marja in the last two weeks, according to the Afghan Red Crescent Society in the provincial capital, Lashkar Gah.