General Dan McNeill of the United States has taken command of 35,500 Nato-led soldiers after nine months of British command.
The transition came after a year of sharply increased violence following the alliance's push into the Taliban's southern heartland.
Military officials expect Gen McNeill, the highest ranking US general to lead troops in Afghanistan, to take a harder line with militants than his predecessor, Gen David Richards.
Gen Richards backed a controversial peace deal in the southern town of Musa Qala that crumbled on Thursday when an estimated 200 Taliban fighters overran the town.
About 4,000 people died in insurgency-related violence last year, according to an Associated Press count based on numbers from US, Nato and Afghan officials.
AP
