Afghanistan's government had complained that foreign forces had jailed many Afghans on dubious grounds and without a proper judicial process.

The release of the prisoners on Thursday was met with opposition from both Britain and NATO-led forces over concerns they could pose a security threat.

"Insurgents in this group of detainees have caused casualties amongst coalition and Afghan forces," the NATO-led force in Afghanistan said in a statement.

The Ministry of Defence in London declined to comment.

At least 55 prisoners released

The Reuters news agency reported 77 prisoners were freed in total, and that more than half were believed to have been captured by British forces. Elsewhere, the German dpa news agency said 55 people had been released from Bagram.

Last month, at least 60 alleged "Taliban fighters" were freed through a decree by Afghan President Hamid Karzai.

The move further damaged Washington-Kabul relations, which were already strained over Karzai's refusal to sign a deal that would keep US troops in the country beyond 2014.

US and allied forces invaded Afghanistan soon after the September 11, 2001 attacks.

Karzai orders all cases to be reviewed

Karzai instructed a special committee, known as the Afghan Review Board, to examine the cases of hundreds of prisoners deemed not guilty of any crime.

Most of the estimated 890 under review had already been released on the grounds of insufficient evidence to try them in court.

"We reviewed 104 cases in the past four months. There wasn't enough evidence and documents to prove the guilt of these [55] inmates, so we released them," said Abdul Shakoor Dadras, who heads the government panel.

Bagram prison was transferred to Afghan control last year as a milestone in the withdrawal of US and coalition forces from Afghanistan by the end of 2014.

lw/msh (Reuters, dpa)