KABUL, Afghanistan (AP) – 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.

The heaviest clash was reported near Tirin Kot in southern Uruzgan province, where suspected Taliban fighters attacked an army convoy late Thursday, sparking three hours of fighting, Gen. Rehmatullah Raufi said. He said the army suffered no casualties.

Elsewhere, militants on a motorcycle opened fire on three men working for an Afghan aid group Thursday, killing two and wounding one, police spokesman Sher Jahn Durani said.

The aid workers were helping farmers in northern Balkh province. It was unclear who was behind the attack, but fighters of Islamist warlord Gulbuddin Hekmatyar are active in that area.

Officials said gunmen also killed the security chief of western Farah province Thursday, while militants ambushed a border police battalion commander in neighboring Herat province, killing him and three bodyguards.

Officials said three Afghan soldiers were killed when their convoy hit a mine or roadside bomb in eastern Paktia province, and a gunbattle between suspected militants and Afghan soldiers north of Kabul killed three militants and wounded four.

The fighting coincided with the release of a statement purportedly from Taliban leader Mullah Omar, who mourned the death of Abu Musab al-Zarqawi after a U.S. airstrike in Iraq and vowed to keep fighting in Afghanistan “against crusaders.” Omar has been hiding since the invasion.

Separately, al-Qaida’s No. 2 praised al-Zarqawi in a videotape broadcast Friday but did not mention his death, suggesting the tape was made earlier.

Ayman al-Zawahri focused on political developments in the Palestinian territories, Sudan and Egypt in the tape broadcast by the Al-Jazeera network. The video is the sixth al-Zawahri has issued this year.

The 33 prisoners released from the U.S. prison had been held for up to 22 months, said Syed Sharif Yousafi, spokesman for a national reconciliation commission. Officials often release detainees deemed to pose no threat.

Some of those freed said they held protests several times in recent months, and U.S. soldiers responded with an unidentified gas that induced fatigue and made breathing difficult.

Former inmate Mohammed Rahim, 40, said the gas was used to halt protests by prisoners demanding to know why they were being held.

“First it feels difficult to breathe, everyone lies on the ground and you feel so tired,” he said.

Abdul Rahman, 50, also said the gas was used to quell protests, most recently a month ago.

Rahman said Americans wearing masks entered and sprayed gas from a tank, hindering breathing and making inmates drowsy. He said the troops gave water to inmates afterward to revive them, but three inmates required medical treatment after the last protest.

The U.S. military has not allowed human rights groups access to Bagram but maintains the roughly 500 prisoners are treated humanely.

Asked if troops have used any type of gas, U.S. military spokeswoman Lt. Tamara D. Lawrence said the military has not received any formal complaints and that the Bagram facility is visited regularly by the International Committee of the Red Cross.