The massive jailbreak overnight in Afghanistan's second-largest city serves as a reminder of the Afghan government's continuing weakness in the south, despite an influx of international troops, funding and advisers. Kandahar city, in particular, has been a focus of the international effort to establish a strong Afghan government presence in former Taliban strongholds.

The 1,200-inmate Sarposa Prison has been part of that plan. The facility has undergone security upgrades and tightened procedures following a brazen 2008 Taliban attack that freed 900 prisoners. Afghan government officials and their NATO backers have regularly said that the prison has vastly improved security since that attack.

But on Sunday night, around 475 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 the majority of the missing were militants.

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

He said more than 500 inmates were freed, and that about 100 of them were Taliban commanders.

Four of those who escaped were provincial-level Taliban commanders, said Qari Yousef Ahmadi, another Taliban spokesman. The highest-profile Taliban inmates would likely not be held at Sarposa.

The U.S. keeps detainees it considers a threat at a facility outside of Bagram Air Base in eastern Afghanistan. Other key Taliban prisoners are held by the Afghan government in a high-security wing of the main prison in Kabul.

A man who Taliban spokesmen said was one of the inmates who helped organize the escape from the inside said a group of inmates obtained copies of the keys to the cells ahead of time.

"There were four or five of us who knew that our friends were digging a tunnel from the outside," said Mohammad Abdullah, who said he had been in Sarposa prison for two years after being captured in nearby Zhari district with a stockpile of weapons. "Some of our friends helped us by providing copies of the keys. When the time came at night, we managed to open the doors for friends who were in other rooms."

He said they woke the inmates up four or five at a time to get them out quietly. Abdullah spoke by phone on a number supplied by a Taliban spokesman. His account could not be immediately verified.

The governor of Kandahar province confirmed at least 475 escaped and said that a search operation is going on to recapture them.

"Some of the prisoners have already been recaptured," Gov. Tooryalai Wesa said. He did not provide further details.

Asked how the tunnel was dug without anyone noticing, Wesa said only that the incident was still under investigation.