KANDAHAR, Afghanistan - More than 400 inmates - many of them Taliban insurgents - escaped from the main prison in the southern Afghan city of Kandahar overnight through an underground tunnel, an official said Monday.

The massive jailbreak in Kandahar, the focus of much of the international military effort to defeat the insurgency, is a reminder that the Afghan government is still weak and easily thwarted in the south, despite an influx of international troops, funding and advisers.

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The escape comes after years of security upgrades and tightened procedures at the 1,200-inmate Sarposa Prison following a brazen 2008 Taliban attack that freed 900 prisoners.

On Sunday night, about 476 prisoners streamed out of a tunnel dug between the prison and the outside and disappeared into Kandahar city, prison supervisor Ghulam Dastagir Mayar said. He said many of the missing were Taliban militants.

Taliban spokesman Zabiullah Mujahid said the insurgents on the outside dug the 1,050-foot (320-meter) tunnel to the prison over five months, bypassing government checkpoints. The tunnel finally reached the prison cells Sunday night, and the inmates were led through it to freedom by three prisoners who had been informed of the plan, Mujahid said.

About 100 of those who escaped were Taliban commanders, and most of the others were fighters with the insurgency, he said.

In the 2008 attack, dozens of militants on motorbikes and two suicide bombers assaulted the prison. One suicide bomber set off an explosives-laden tanker truck at the prison gate while a second bomber blew up an escape route through a back wall. About 900 inmates escaped, including 400 Taliban fighters.