"We're starting to see them come back because the Taliban leadership is furious that it lost so much ground and they are actively trying to reassert their control," Lt. Col. Jason Morris, commander of the Marine battalion in the area, told the Los Angeles Times.
The newspaper reported Tuesday developments in Sangin could be a litmus test of the durability of the gains U.S. forces have made since the surge in troops last year.
"The gains are fragile and reversible," Defense Secretary Robert Gates told reporters during a surprise visit to the country this week. "If we can sustain the gains we've made and expand them further, I think it'll be a powerful message."
The Times noted Afghan President Hamid Karzai has voiced less-than-sterling support for the surge strategy.
"We are grateful to the international community," he said in a recent speech in Kabul. "But they haven't brought us peace."
Karzai has said he wants to see an end to joint civil-military provincial reconstruction teams that focus on development projects. He also has suggested there is no long-term military solution to the Afghan conflict.
The United States is aiming to begin a slow pullout of troops this year, but expects to have forces in Afghanistan at least until 2014.