US Gen Stanley McChrystal said Nato troops must make a "cultural shift" away from being a force designed for high intensity combat and instead make protecting Afghan civilians their first priority.
The newly arrived four-star commander said he hopes to install a new military mindset by drilling into troops the need to reduce the number of Afghan civilians killed in combat.
Gen McChrystal is expected to formally announce new combat rules within days that will order troops to break away from fights - if they can do so safely - if militants are firing from civilian homes. One effect of the new order will be that troops may have to wait out insurgents instead of using force to oust them, he said.
"Traditionally American forces are designed for conventional, high-intensity combat," Gen McChrystal said during a visit to Camp Leatherneck, a new US Marine base housing thousands of newly deployed Marines in southern Helmand province.
"In my mind what we've really got to do is make a cultural shift."
Because the military is such a big organisation, the new message will take "constant repetition," he said.
President Hamid Karzai has pleaded with Nato forces for years to reduce the number of Afghan villagers killed in combat.
Mr Karzai has long said that such deaths turn civilians away from the government and international forces and toward the Taliban, a point Gen McChrystal underlined.
"When you do anything that harms the people you just have a huge chance of alienating the population," he said. "And so even with the best of intentions, if our operation causes them to lose property or loved ones, there is almost no way somebody cannot be impacted in how they view the government and us, the coalition forces."
Gen McChrystal, who took command of all Nato troops in Afghanistan last week, is making his first visits to regional commanders to outline the new combat rules.
Brig Gen John Nicholson, the Marine commander at Camp Leatherneck, said his forces were already following Gen McChrystal's new commands.
"Our focus from the very beginning has not been Taliban. It's been civilians," he said. "We've paid a lot of attention to avoiding civilian casualties. ... We have a lot of combat vets, a lot of Iraq vets. And I think we learned early on the importance of trust and support of the locals."
He added: "There will be plenty of opportunities to kill Taliban, and we're pretty good at that. Bur the focus here, the reason we're here, is the people, not the Taliban."
The Pentagon has asked Gen McChrystal for a 60-day review of the Afghan war, a review that could result in a recommendation to shift troops to new locations in Afghanistan. Gen McChrystal said he didn't yet know if he would request more troops.
The Pentagon abruptly pulled McChrystal's predecessor - Gen David McKiernan - out of Afghanistan one year into a two-year assignment. Gen McChrystal said his deployment did not have a timetable to it, and that he would stay in Afghanistan as long as the Pentagon wanted him there.
He refused to give even an estimate of how long that might be, saying: "My wife would kill me if she read something too long. I do think continuity is key, though."