(Adds Pentagon on Tora Bora operation, paragraphs 6-8)

KABUL, Aug 23 (Reuters) - Taliban insurgents killed 10 Afghan security guards escorting a logistics convoy for U.S. forces in an ambush in southern Afghanistan on Thursday, the head of the security detail said.

Mohammad Salim said the attack took place on the main highway that runs through the province of Zabul.

Elsewhere, a roadside bomb killed three Afghan civilians and wounded 14 others in the southern province of Helmand, the local police chief said.

The Afghan Ministry of Defense also said on Thursday that 12 Taliban fighters and one soldier had been killed in clashes across the country.

Afghanistan is witnessing its worst period of violence since U.S.-led and Afghan forces toppled the Taliban in 2001.

In the United States, a U.S. general said U.S. and Afghan forces may have inflicted 100 casualties on their enemies in operations this month against al Qaeda and other militants in the Tora Bora mountains near the border with Pakistan.

"Those operations have concluded and the battlefield assessments are still ongoing," U.S. Army Brig. Gen. Richard Sherlock told reporters. "Our initial assessments indicate that those operations inflicted approximately 100 casualties.

"Combined follow-on operations will continue in that area," said Sherlock, an officer on the U.S. military's Joint Staff.

AlertNet news is provided by