Afghan soldier stands guard outside the main prison in Kandahar, south of Kabul, Afghanistan on Monday, April 25, 2011. Taliban insurgents dug a more than 1,050-foot (320-meter) tunnel underground and into the main jail in Kandahar city and whisked out more than 450 prisoners, most of whom were Taliban fighters, officials and the insurgents said Monday.

KANDAHAR, Afghanistan — Taliban militants dug a lengthy tunnel underground and into the main jail in Kandahar city and whisked out more than 450 prisoners, most of whom were Taliban fighters, officials and insurgents said Monday.

The massive overnight jailbreak in Afghanistan's second-largest city underscores the Afghan government's 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, about 475 prisoners streamed out of a tunnel that had been dug into the facility and disappeared into Kandahar city, prison supervisor Ghulam Dastagir Mayar said. He said the majority of the missing were Taliban militants.

“This is a blow,” presidential spokesman Waheed Omar said. “A prison break of this magnitude of course points to a vulnerability.” He did not provide details on the incident, saying that the investigation had just started.

The prison break also weakens the argument that the international troops are making good progress in handing over more responsibility for security to Afghan forces, which will eventually enable the coalition to leave. President Barack Obama plans to start drawing down forces in July.

The Kandahar escape is the latest in a series of high-profile Taliban operations that show the insurgency is fighting back strongly against the surge of U.S. and NATO forces. Over the past year, tens of thousands of U.S. and NATO reinforcements routed the Taliban from many of their southern strongholds, captured leading figures and destroyed weapons caches.

The militants have responded with major attacks across the nation as the spring fighting season has kicked off. In the past two weeks, Taliban agents have launched attacks from inside the Defense Ministry, a Kandahar city police station and a shared Afghan-U.S. military base.