Gunmen on a motorbike killed two Afghan aid workers and government forces battled Taliban militants in the restive south as 26 people died in violence across Afghanistan, officials said Friday.

Meanwhile, a purported statement from Taliban chief Mullah Omar mourned the death of al-Qaeda's leader in Iraq and vowed to keep fighting "crusaders" in Afghanistan.

The heaviest clash was reported near Tirin Kot in southern Uruzgan province, where suspected Taliban attacked an Afghan army convoy late Thursday, sparking three hours of fighting that killed 13 militants, said regional army commander Gen. Rehmatullah Raufi. He said the army suffered no casualties.

Raufi claimed soldiers found Pakistani identity cards on the bodies of two militants.

CountryWatch: Afghanistan

Afghan officials claim militants, including Taliban leaders, find sanctuary inside neighboring Pakistan, a charge Islamabad denies.

Taliban militants have stepped up attacks across the volatile south this spring, and in the past three weeks alone, more than 400 people have died in violence, raising new fears for the war-battered country's future, four-and-a-half years after a U.S.-led invasion ousted the hardline regime for hosting al-Qaeda leader Usama bin Laden.

Also Thursday, militants on a motorbike opened fire on three men working for an Afghan aid group helping farmers in northern Balkh province, said Sher Jahn Durani, spokesman for the provincial police chief. Two of the aid workers died and a third was wounded.

It wasn't immediately clear who was behind the attack, but fighters of anti-government Islamist warlord Gulbuddin Hekmatyar are active in the area.

On Friday, a written statement purportedly from fugitive Taliban leader Mullah Omar expressed "deep sadness" over the death of al-Qaeda in Iraq leader Abu Musab al-Zarqawi.

The Pashto-language statement said al-Zarqawi's death — in a U.S. airstrike near Baghdad on Wednesday — would not weaken the resistance in Iraq, "as it is the people's resistance and every youth can become al-Zarqawi."

"I want to assure the Muslims across the world that we will not stop our struggle against crusaders in Afghanistan," Omar was quoted as saying.

The authenticity of the statement could not be confirmed. It was forwarded to an Associated Press reporter in Pakistan by e-mail from Dr. Mohammed Hanif, who often issues statements on behalf of the Taliban, although his exact ties to the group's leadership are unclear.

Omar has been at large since the Taliban's ouster, and is thought to be hiding in southern Afghanistan or in the rugged Pakistan-Afghan border region.

In other violence reported across Afghanistan, two gunmen shot dead the security chief of western Farah province as he walked in a market in the town of Bala Buluk on Thursday. In neighboring Herat province, militants ambushed the commander of a battalion of border police, killing him and three bodyguards, officials said.

Also Thursday, three Afghan soldiers were killed when their convoy hit a mine or roadside bomb in eastern Paktia province, said Gen. Murad Ali, a regional corps commander.

In Kapisa province, north of Kabul, a gunbattle between suspected militants and Afghan soldiers on Thursday killed three militants and wounded four, said Gen. Zahir Azimi, the Defense Ministry spokesman. One militant was arrested.

Meanwhile, 33 Afghans, arrested during security operations in southern and eastern Afghanistan then held for between four and 22 months, have been freed from detention the main U.S. base in Afghanistan, said Syed Sharif Yousafi, spokesman for a national reconciliation commission.

Some of inmates freed Thursday from Bagram Air Base said there had been several protests in recent months at the prison over their indefinite detention, and American soldiers had suppressed them using an unidentified gas that briefly induced fatigue and made it difficult to breathe.

Abdul Rahman, 50, said there had been three protests during his 11 months at Bagram. He claimed the gas used by American soldiers in his cage holding 18 to 20 men had caused all the inmates to feel tired and lie down, but they revived when given water.

The U.S. military did not respond Friday to a request for comment on the allegations. It has previously said inmates are treated humanely and in accordance with the principals of the Geneva Convention.