An assassin struck at the heart of President Hamid Karzai's political machine in southern Afghanistan Wednesday, killing the mayor of Kandahar with an exploding turban and deepening a power vacuum in the Taliban's main stronghold.

The slaying of Mayor Ghulam Haider Hamidi was the third killing of a Karzai associate in a little more than two weeks. The attacks have jeopardized the Afghan government's tenuous grip on the strategic south after recent success in routing the Taliban.

On July 12, a close associate gunned down Karzai's powerful half brother at his home in Kandahar. Five days later, Karzai's inner circle suffered another hit when gunmen in Kabul killed Jan Mohammad Khan, a presidential adviser on tribal issues and a former governor of Uruzgan province, which is also in the south.

The 65-year-old, gray-haired mayor was slain inside a heavily fortified government compound just before he was to meet with local residents caught up in a land dispute, according to Mohammad Nabi, an employee of the mayor's office. The attacker was holding a piece of paper and trying to talk to the mayor when he detonated a bomb hidden inside his turban, said Nabi, who witnessed the killing.