KABUL — The Taliban has amassed hundreds of fighters in Helmand Province in its most ambitious attempt this year to seize ground as the U.S. combat mission winds down in Afghanistan, according to Afghan officials.

The fighting has gone on for nearly a week in the southern province, a former Taliban stronghold, demonstrating the insurgency’s ability to mobilize against much improved, but still inexperienced, Afghan security forces. As of Saturday, the exact toll from the violence remained unclear, but at least several dozen civilians, soldiers and police officers have been killed, Afghan officials said.

“It was a coordinated and organized attack on northern Helmand,” said Omer Zowak, the provincial spokesman. “Afghan security forces have pushed the Taliban back . . . but they are resisting in some parts of Sangin district.”

Sangin, the main target of the Taliban offensive, lies in a valley along the Helmand River where for years insurgents have staged periodic attacks on Afghan and international troops. This week, 45 civilians were killed in Sangin alone and 4,000 more were displaced, according to the district’s governor, Sulaiman Shah.

“The Taliban booby-trapped people’s home, putting bombs in front of their houses,” Shah said. “The situation was very bad.”

When U.S. troops began to leave Helmand last year, many American officials predicted that the Taliban would attempt to dominate the Afghan forces left in charge of Sangin. For years, the district’s poppy harvest and drug production centers were a financial boon to insurgents and the Taliban appeared to make retaking Sangin a priority.

Last week’s fighting spread to the districts surrounding Sangin, prompting reinforcements to be sent to the area. “The clearing operation is going on,” said Gen. Zahir Azimi, a spokesman for the Afghan National Army. “We’ve killed hundreds of Taliban fighters.”