Taliban fighters reportedly have captured their first provincial capital Friday since U.S. and NATO troops began pulling out of Afghanistan: the city of Zaranj in southern Nimroz province. 

The group has posted images on social media showing insurgents inside the local airport and posing for photographs at the entrance to the city, according to the Associated Press. Nimroz is sparsely populated in a region that’s mainly desert and Zaranj has about 50,000 residents. 

"Taliban have the control of Zaranj city," Rohgul Khairzad, the deputy governor of Nimroz province, told NBC News in a Facebook message, noting that it fell around 4 p.m. local time. 

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Khairzad added that she "encourage[s] the residents of Nimroz to try to stay indoors until we see what the Taliban are planning to do." 

Fox News has reached out to the U.S. State Department for comment. 

The province's governor, Abdul Karim Barahawi, has fled Zaranj for refuge in the peaceful Chahar Burjak district, where the local ethnic Baluch population has given him protection, the Associated Press reports. 

The Taliban have been surging for months in Afghanistan, taking swaths of land as U.S. and NATO forces complete their final pullout from the country by the end of the month. 

Meanwhile, Afghan and U.S. aircraft pounded Taliban positions in southern Afghanistan’s Helmand province on Friday, as the insurgents closed a major border crossing with neighboring Pakistan. 

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Residents in Helmand's contested provincial capital, Lashkar Gah, said airstrikes destroyed a market in the center of the city — an area controlled by the Taliban. Afghan officials say the Taliban now control nine out of the 10 districts of that city. 

Afghanistan’s elite commandos have deployed to Lashkar Gah, backed up by airstrikes by the Afghan and U.S. air forces. 

More than half of Afghanistan's 421 districts and district centers are now in Taliban hands, according to the Associated Press. While many of the districts are in remote regions, some are deeply strategic, giving the Taliban control of lucrative border crossings with Iran, Tajikistan and Pakistan. 

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At a special meeting at the U.N Security Council on Friday, Deborah Lyons, the U.N. envoy to Afghanistan, said the human toll of the worsening fighting was deeply disturbing. 

"The war in Afghanistan has entered a new, deadlier, and more destructive phase," she said. "The provincial capitals of Kandahar, Herat, and Lashkar Gah in particular have come under significant pressure. This is a clear attempt by the Taliban to seize urban centers with the force of arms." 

The Associated Press contributed to this report.