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KABUL—A Taliban insurgent driving a vehicle packed with explosives slammed into a U.S. military convoy in Kabul on Saturday, killing at least 13 Americans inside an armored vehicle, according to Western officials and eyewitnesses in Afghanistan.

The midday attack, which also killed at least four Afghans, was believed to be the deadliest strike on American forces in the relatively secure Afghan capital in a decade of war.

NATO's International Security Assistance Force said that those killed were five ISAF soldiers and eight civilian employees.

U.S. Gen. John R. Allen, commander of coalition forces in Afghanistan, said he was "saddened and outraged" by Saturday's attacks. "The enemies of peace are not martyrs, but murderers," he said.

The Kabul bombing was the worst in a series of attacks across Afghanistan on Saturday, showcasing the insurgency's resilience despite recent coalition assertions of reversing the war's momentum.

In the southern Uruzgan province, an Afghan army officer opened fire on his Western comrades, killing three Australian soldiers and an Afghan interpreter, according to the U.S.-led coalition and Western officials.

The attacks came a day after a new Pentagon report hailed a summer drop in violence as a sign that the insurgency is losing steam. Top military officials in Afghanistan argued that President Barack Obama's military surge of 30,000 extra forces had reversed the Taliban momentum and given the U.S.-led coalition a new advantage.