MOHAMMAD AGHA, Afghanistan, (Reuters) - 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 a new offensive a week ago in the Taliban bastion of Helmand. The blast happened in Logar province south of Kabul. Officials said it was caused by explosives hidden beneath a pile of firewood in the back of 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. The blast went off as police checked the abandoned truck in the morning, he said. 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. Reuters television pictures showed villagers digging through the rubble of destroyed shops, and a large Mercedes truck blown onto its side by the force of the blast. Twisted bicycles lay among the debris. Nearby, relatives wept over the bodies of two children covered by white sheets under the shade of apple trees, the ground strewn with fallen fruit. President Hamid Karzai condemned the blast as a savage and anti-Islamic attack, the palace said in a statement.