(CNN)-- Earlier this week, a lone Taliban gunman with a red skull cap and a rifle sauntered up to the gates of Mazar-i-Sharif, Afghanistan's fourth largest city, and snapped a selfie.

The last time the Taliban controlled the city was 20 years ago, when they left hundreds of captives in steel trucking containers to suffocate and die in the scorching desert heat.

Now, the militants are back at the city gates once again, as part of a lightning offensive against Afghan government forces that has set alarm bells ringing from Kabul to Washington.

One by one, the Taliban has been taking over areas in a number of provinces in northern Afghanistan in recent weeks, local officials told CNN. The Taliban says it has taken control of 90 districts across the country since the middle of May. Some were seized without a single shot fired.

The UN's special envoy on Afghanistan, Deborah Lyon put the figure lower, at 50 out of the nation's 370 districts, but told the Security Council on Tuesday she feared the worst was yet to come.

"Most districts that have been taken surround provincial capitals, suggesting that the Taliban are positioning themselves to try and take these capitals once foreign forces are fully withdrawn," Lyon said.