Kabul, Afghanistan ? Suspected Taliban militants attacked an Afghan army convoy, sparking hours of fighting that killed 13 rebels during an unrelenting wave of violence that claimed at least 26 lives, officials said Friday.

In a separate incident, gunmen on a motorcycle killed two Afghan aid workers.

The latest bloodshed came as the U.S. military released 33 Afghans from a prison at Bagram Air Base. Some of those prisoners accused U.S. troops Thursday of putting down protests with an unspecified gas that made inmates drowsy and restricted their breathing. The military said it had received no formal complaints.

Taliban militants have stepped up attacks across the volatile south, and in the past three weeks more than 400 people have been killed. The attacks have raised new fears for the war-battered country’s future more than four years after a U.S.-led invasion ousted the hard-line Taliban regime for harboring Osama bin Laden.