KABUL: Afghan authorities prepared Saturday to retake a key border crossing seized by the Taliban in a sweeping offensive that the insurgents claim has helped capture a vast swath of the violence-wracked nation. As US troops continued their withdrawal, the Taliban said its fighters had seized two crossings in western Afghanistan — completing an arc of territory from the Iranian border to the frontier with China. It now held 85 percent of the country, a Taliban official said Friday, controlling about 250 of Afghanistan”s nearly 400 districts — a claim impossible to independently verify, and disputed by the government. Beijing, meanwhile, which has criticized Washington for its hasty withdrawal, urged its citizens to leave the country “as soon as possible” after evacuating 210 nationals. The “complex and severe domestic security situation” prompted the evacuation warning, the foreign ministry said, adding that 22 of those flown out tested positive for coronavirus on arrival in China.