KABUL, Afghanistan — The Taliban have captured the capital of one of Afghanistan’s western provinces, Afghan officials said on Friday, a symbolic milestone in the insurgents’ relentless march to retake power in the country.

       Taliban fighters faced little resistance in taking Zaranj, the provincial capital of Nimruz on the Afghanistan-Iran border, Afghan officials said. They said a deal had been negotiated with the Taliban allowing the authorities in the city to flee across the border to Iran with their families.

       The assault marked the first provincial capital to be captured by the insurgent group since the Biden administration said it would completely withdraw American troops from Afghanistan by next month.

       The seizure of Zaranj is a symbolically significant development in the Taliban’s campaign, as they moved away from targeting rural districts to focus on attacking provincial capitals, with Zaranj being the first to fall.

       The 215th Corps of the Afghan National Army is responsible for security in both Zaranj and Lashkar Gah, the capital of neighboring Helmand Province, which has been under siege for several days. The 215th Corps’ leadership had shifted its focus to defending Lashkar Gah, leaving Zaranj, a city of 160,000 people, vulnerable to capture.

       “All the people are hiding in their houses in fear of the Taliban,” said Khair-ul-Nisa Ghami, a member of the provincial council. “The situation is very worrying. People are scared,” she said, adding: “The Taliban captured the city without any fighting.”

       The Taliban have besieged a host of such cities for weeks, and Zaranj’s likely fall on Friday is the Taliban’s first major breakthrough. The city’s collapse came as the insurgents also pressed hard into other provincial cities, in a day of bleak news for the government.