Pakistan on Wednesday denied a Taliban claim that it had failed to free former rebel commander Abdul Ghani Baradar, whose release was meant to boost Afghanistan's peace process.

Baradar, often described as the Taliban's former second-in-command, was supposedly set free last month, after months of negotiations between the two governments.

"However, with great regret, he is still spending days and nights locked up behind bars in worrisome health conditions which are deteriorating by the day," the Taliban said in a statement on their website.

A senior Taliban member told AFP that Baradar was being held at a house in Karachi run by Inter-Services Intelligence (ISI), Pakistan's leading intelligence agency.

"He doesn't have any freedom, and his family can't even visit him," he said. "The Pakistan government says he has health problems which are being treated, and then his family will be able to visit."

A separate Taliban source alleged that the ISI was trying to "soften up" Baradar so that he would play a role in the Afghan peace process that may benefit Pakistan.

Pakistan foreign ministry spokesman Aizaz Ahmed Chaudhry insisted Baradar had been released.

"Pakistan has released Taliban detainees to facilitate the reconciliation process in Afghanistan. Mullah Baradar has been similarly freed," he said in a text message to AFP late Wednesday.

"As far as we are concerned, he is free to meet and contact anyone to advance the cause of reconciliation."

Baradar has been touted by some as an influential Taliban voice who could persuade the militants to end the bloody insurgency they have waged since being ousted from power in 2001.

A security source in Pakistan confirmed Baradar's house arrest, which would set back efforts to bring peace to Afghanistan as NATO troops withdraw by the end of next year.

"He is in protective custody in an ISI house in Karachi, he cannot meet anybody or move anywhere on his own, he needs permission of security officials," the source said.

"He has the freedom of having the food of his own choice, but he is restricted to the safe house."

The Afghan government has long demanded that Pakistan free Baradar, who was arrested in Karachi in 2010.

At the time of his detention, Baradar was reported to have been the right hand man of the supreme commander Mullah Omar.

But some analysts say he has lost influence with the current Taliban high command and would have little effect on the current peace efforts.