The Taliban on Wednesday rejected Afghan President Ashraf Ghani's offer to release 1,500 insurgent prisoners ahead of peace talks, saying it wanted 5,000 captives freed before opening negotiations.

Taliban political spokesman Suhail Shaheen told AFP the prisoners should be released "as a trust-building measure" before intra-Afghan talks.

Any changes amounted to a violation of the deal struck between the insurgents and Washington in Doha last month, he added.

Although Kabul was not a signatory to the deal, it stated that up to 5,000 Taliban prisoners held by Afghan authorities would be released, prompting an angry reaction from Ghani.

Shaheen's announcement came hours after Ghani issued a decree saying the government would release 1,500 captives starting Saturday -- but only if the insurgents cut violence.

Another 3,500 prisoners would be released after talks started, a spokesman said on Twitter.