"I urge the two sides to sit down immediately for talks on this issue in Doha, Qatar to work out the details," Zalmay Khalilzad, the US negotiator, said on Twitter.

"The Afghan government has agreed to do so. When implemented, this will be a significant step in the peace process."

The prisoner release was a key element of a deal signed on February 29 in Qatar between the Taliban and the United States, which has started pulling troops out of Afghanistan under that agreement aiming to end its longest-ever war.

After initial hesitation, President Ashraf Ghani's administration said it would release 1,500 Taliban prisoners as a goodwill gesture starting Saturday, with another 3,500 to be freed after negotiations begin.

The Taliban plan to free 1,000 captives.

First-ever negotiations between the Afghan government and the Taliban failed to open Tuesday in Norway as had been the goal under the deal.

But the State Department voiced optimism, attributing the delay to the political crisis in Kabul where Ghani's rival Abdullah Abdullah has also declared himself president following disputed elections.

"President Ghani has told us he is consulting with Dr Abdullah and other Afghan leaders and will announce an inclusive team in the coming few days," State Department spokesman Morgan Ortagus said.

Ortagus, in line with Khalilzad, said that the level of violence by the Taliban was "unacceptable."

"We acknowledge the Taliban have taken steps to stop attacks against the Coalition and in cities. But they are killing too many Afghans in the countryside," she said.