United States special envoy Zalmay Khalilzad has undertaken extensive recent peace talks with the Taliban about ending the 17-year war in Afghanistan, with another round expected in late February.

The expanded negotiating team unveiled by the insurgents includes Anas Haqqani, who was captured in 2014 and whose older brother is deputy Taliban leader and head of the Haqqani network.

A spokesman for President Ashraf Ghani, whose administration was not invited to the most recent round of Taliban talks in January, flatly denied Anas would be freed.

"Anas Haqqani is in prison, and no decision has been taken for his release," the spokesman, Haroon Chakhansuri, said on Twitter.

"The people of Afghanistan can rest assured that justice will be ensured."

The Taliban has long demanded that Anas be released, saying he is a student. Afghan authorities accuse him of a being a high-level player in the Haqqani network.

The Taliban affiliate is a designated terror group by Washington that has been blamed for some of the most shocking and brutal attacks across Afghanistan since the US invasion of 2001.

"Anas Haqqani was captured by the Americans, and should be released to better help with the talks," Taliban spokesman Zabihullah Mujahid told AFP via WhatsApp.

The insurgents said the team announced Tuesday "would continue the current peace talks with the Americans".

Khalilzad, who last met the militants in January, has said he hopes to strike a deal with the Taliban before the Afghan presidential elections in July.

But he has urged caution, saying he did not trust America's long-time adversary and that major hurdles remain.

The special envoy has stressed that any US troop withdrawal would be dependent on conditions on the ground, and that the Taliban must sit down with Kabul and come to an agreement.

The Taliban refuse to talk to Ghani's government in Kabul, which they consider a US puppet.

Khalilzad is leading a large delegation on a six-nation tour, including Afghanistan, to boost the peace process.

He and other US negotiators are expected to meet the Taliban in Qatar, where the insurgents have their political office, later this month.

The insurgents, who were toppled by US-led forces in 2001, last week held separate talks in Moscow with a senior delegation of Afghan politicians, including chief Ghani rivals.

The two-day talks were the Taliban's most significant engagement with Afghan leaders in years, though without the involvement of the government it was unclear what impact they will have.