KABUL, Afghanistan — U.S.-backed Afghan forces launched a massive manhunt Monday for hundreds of inmates, most of them Taliban fighters, who fled down a tunnel burrowed under the walls of southern Kandahar Province’s main maximum-security prison.

The second spectacular mass breakout from Sarposa Prison in nearly three years showed that Afghan security forces remain plagued by deficiencies despite billions of dollars in training and equipment from the United States and other countries.

The U.S. and its allies are working to bolster the country’s security forces as a major component of their strategy to transfer responsibility for security throughout the country to the national government by 2014.

“This incident reminds that we still have loopholes,” conceded Wahed Omar, a spokesman for President Hamid Karzai. “We are worried about it. This is a blow and it is something that should not have happened.”

The breakout, an intensifying campaign of assassinations of local officials and a recent series of attacks on government facilities show how resilient the Taliban-led insurgency remains despite suffering battlefield setbacks since last year’s surge of U.S. forces into Kandahar and the adjacent insurgent stronghold of Helmand province.

At least 488 prisoners fled Sarposa Prison though the 1,050-foot tunnel dug from a nearby house before guards in the facility, which is encircled by walls topped with razor wire and guard towers, discovered the breakout.

Wesa accused Afghan intelligence and prison officials of neglect and of failing to properly perform their duties.

Afghan security forces closed off exits from Kandahar city, the headquarters of the Taliban movement until the 2001 U.S. invasion, and fanned out in a search of the escapees, rounding up 24 by the end of the day.