Taliban insurgents tightened their grip on Afghanistan on Friday, wresting control of its second and third biggest cities while Western embassies prepared to send in troops to help evacuate staff from the capital, Kabul.

The capture of the second-biggest city of Kandahar in the south and Herat in the west after days of clashes are a devastating setback for the government as the deadly Taliban insurgency turns into a rout of the security forces.

"The city looks like a...