Shafaq News/ The Taliban seized the capital of an Afghan province along the Iranian and Pakistani borders on Friday, the first such takeover since the insurgents launched an offensive to take advantage of the U.S. troop withdrawal.

Expanding their assassination campaign in Kabul, the militants Friday also gunned down the head of the Afghan government’s media office.

The Taliban have swept through much of Afghanistan’s countryside since President Biden announced the withdrawal in April. Though the Taliban have pierced the defenses of some provincial capitals during this offensive, Friday’s fall of Zaranj, the capital of Nimroz in southwest Afghanistan, marked a major milestone.

Videos circulating on social media showed Taliban fighters loitering near the open gates of the Nimroz governor’s residence and others sitting atop captured Humvees. Mohammad Yonus Arab, a provincial council member in Nimroz, said the Taliban attacked the city from three directions and its defenses collapsed quickly. The governor and senior officials fled to the adjacent Chahar Borjak district, the only one in the province still under government control, Mr. Arab said.

Mr. Arab, who also fled to Chahar Borjak, said the entire city is now controlled by militants and government offices were being looted. A resident of Zaranj, reached by telephone, said the Taliban had taken the city without a fight. She asked that her name not be used for fear of retaliation.

“I am afraid,” she said. “The Taliban are in the city and who knows if they come and search houses.”

Though sparsely populated, Nimroz is an important trade gateway with Iran. In recent weeks, the Taliban captured most of Afghanistan’s international border crossings, leaving only a handful of customs points under tenuous government control.

While the Taliban had seized some Afghan provincial capitals in the past, such as the city of Kunduz in 2016, they were quickly pushed out at the time thanks to U.S. air support—backing that is no longer readily available because all American combat aircraft have now been withdrawn from the country. For now, the U.S. continues limited air missions from bases in the Persian Gulf.

A U.S. defense official said he was unaware of U.S. airstrikes leading up to the fall of Zaranj. Recent U.S. airstrikes had focused on Lashkar Gah, Herat and the city of Kandahar, where Afghan forces were engaged with Taliban, he said.

The United Nations said Friday that more than 1,000 civilians were injured or killed in the past month in the fighting around those three major provincial capitals, warning that Afghanistan is now at a “dangerous tipping point” where cities could become all-out war zones.