An explosion outside a school south of the Afghan capital on Thursday killed at least 25 people, including 15 students, officials said, the worst toll from a single blast in a year. It was the latest incident in an escalation of violence across Afghanistan since US Marines launched Operation Khanjar a week ago in the Taliban bastion of Helmand.

The blast occurred in Logar province. Officials said it was caused by explosives hidden beneath a pile of firewood in a truck, which had crashed overnight, prompting speculation they were meant for elsewhere. Logar police chief Ghulam Mustafa said the truck rolled into a stream between two schools.

Mustafa said the explosives were possibly being taken by Taliban insurgents to Kabul for a planned attack. He was not sure if the blast was deliberately aimed at the police in Logar as a secondary target after the truck overturned. The blast went off as police checked the abandoned truck in the morning, he said.