More than 400 inmates, many of them Islamic insurgents, escaped overnight from the main prison in the southern Afghan city of Kandahar, the spiritual home of the Taliban movement, according to NATO officials.
The inmates escaped through a nearly 400-yard tunnel they had spent six months digging, Taliban spokesman Qari Yousef Ahmadi said.
The escape at the prison, which holds 1,200 inmates, began after dark and finished just before daybreak, said British Maj. Tim James, a spokesman for NATO forces in Kabul.
James said Afghan officials, who run the prison and are managing the investigation into the prison break, reported that 476 inmates had escaped, while the Taliban claimed the figure was 541.
"We don't know the exact number or how it happened," James said. "We know there's an operation ongoing to try to recover some of them."
He said North Atlantic Treaty Organization forces were on standby early Monday to assist with tracking down the escaped prisoners.
Kandahar, Afghanistan's second-largest city, has been a hotbed of violence in recent days. On April 15, Taliban insurgents assassinated the Kandahar police chief at his headquarters.
Ahmadi, the Taliban spokesman, claimed that 106 of the escapees were "important commanders" of the Taliban. James said it was unclear whether the prisoners who escaped were leaders likely to mount attacks on coalition troops.
"Any escaped prisoners are always a concern," James said.
The escape is not the first at the Kandahar prison. In June 2008, nearly 900 prisoners broke out amid a well-coordinated attack by Taliban insurgents.
In that escape, an enormous truck bomb blew open the front gate of the prison and a suicide bomber also struck the back of the facility. In the fierce battle that ensued, the militants also fired rockets at the prison.
And in March 2010, more than 30 people were killed as four suicide bombers struck near the prison, a hotel and a police station, possibly in an attempt to free Taliban militants from the prison.
Special correspondent Aimal Yaqubi in Kabul contributed to this report.