By Mohammad Obaid

PUL-I-ALAM, Afghanistan (Reuters) - An explosion outside a school south of the Afghan capital on Thursday killed at least 25 people, including 12 students, officials said.

It was the latest incident in an escalation of violence across Afghanistan since U.S. Marines launched a new offensive a week ago in the Taliban bastion of Helmand.

That assault, Operation Strike of the Sword, is the first major operation under U.S. President Barack Obama's new regional strategy to defeat the Taliban and stabilize Afghanistan.

The blast happened in Logar province south of Kabul and 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.

Explosives are used widely in Afghanistan for legal activities such as logging and construction.

The Taliban have been active for some time in Logar, which lies close to the insurgency's eastern strongholds.

The Marines launched the Helmand offensive last week with insurgency violence at its highest since the austere Islamists were ousted by U.S. and Afghan forces in 2001 for failing to hand over al Qaeda leaders wanted over the September 11 attacks.

VARIETY OF ATTACKS

While there have been no major engagements in Helmand in the past week, Afghan civilians and troops and foreign soldiers have been killed in a variety of attacks across the country.

At least 17 foreign soldiers, most from the United States and Britain, have been killed in the past six days, making it one of the bloodiest weeks for foreign troops for many months.

Brigadier General Lawrence Nicholson, commander of the Marine Expeditionary Brigade in Afghanistan, told Pentagon reporters in Washington overnight his forces had met only light resistance in Helmand so far with about 20 armed confrontations.

The 4,000 Marines taking part in the Helmand operation are part of a wider influx of about 10,000 Marines, the biggest single wave of an escalation of thousands more U.S. troops planned under Obama's new strategy.

U.S. troops levels are planned to reach about 68,000 by the end of the year, more than double the 32,000 at the end of 2008. One of the main goals of the new operation is to capture ground from the Taliban and then hold it, something overstretched British-led NATO troops have so far been unable to achieve. It is also seeking to win over Afghans from the insurgency.

But Nicholson said the Helmand offensive has too few Afghan troops, making it difficult to achieve that latter task.

"What I need is Afghan troops," he said in an audio link from Afghanistan. "I've got 4,000 Marines in the field and about 600, 650 Afghans. You can do the math."

(Additional reporting by Sayed Salahuddin in KABUL and David Morgan in WASHINGTON; Writing by Paul Tait; Editing by David Fox)