The planned exchange of three senior Taliban commanders for an American and an Australian kidnapped in Afghanistan in 2016 has not taken place, a diplomat and a former Afghan official has said.

Key points: Professors Timothy Weeks and US citizen Kevin King were kidnapped by the Taliban in 2016

Professors Timothy Weeks and US citizen Kevin King were kidnapped by the Taliban in 2016 DFAT said the Afghan Government was leading discussions with the Taliban

DFAT said the Afghan Government was leading discussions with the Taliban A spokesman for the Taliban said the hostages would be released only when the released insurgents "reach their destination"

The diplomat, who spoke to reporters on condition of anonymity, declined to provide details about why the planned exchange, which Afghan President Ashraf Ghani announced on Tuesday, did not occur.

The Afghan Government's decision to free Anas Haqqani and two other Taliban commanders, Haji Mali Khan and Hafiz Rashid, was taken in the hope of securing direct talks with the Taliban, which has refused to engage with what it calls an illegitimate "puppet" regime in Kabul. All three were captured in 2014.

In return, Australian citizen Timothy Weeks and US citizen Kevin King, professors kidnapped by the Taliban in August 2016 from the American University of Afghanistan in Kabul, were to be freed.

A spokesperson for the Australian Department of Foreign Affairs and Trade told the ABC, while they have "never stopped pressing" for the release of the two men, they would "not give a running commentary on the current process".

"The Afghan Government is leading the discussions with the Taliban for Mr Weeks' and Mr King's release as part of its broader peace negotiations," the spokesperson said.

Earlier on Wednesday (local time), a spokesman for the Taliban said the hostages would be released only when the insurgents "reach their destination".

"When our captives reach their destination, the American University professors will be released," said spokesman Zabihullah Mujahid.

He did not specify what that destination would be.

Mr Mujahid said he "cannot confirm" if the Taliban prisoners had already been released. They were last known to have been detained by Afghan authorities in Bagram prison, north of Kabul.

President Ghani had hinted on Tuesday that they could be released outside the country, stating that after consulting with international partners including Washington "we have made sure that the release of these three prisoners would not reinforce the enemy's frontlines or increase their attacks".

Timothy Weeks (left) and his colleague Kevin King were kidnapped at gunpoint by the Taliban in 2016. ( Supplied: YouTube )

Mr King and Mr Weeks were kidnapped by gunmen wearing military uniforms in the heart of Kabul in August, 2016.

They later appeared looking haggard in a Taliban hostage video, with the insurgents going on to say that Mr King was in poor health.

On Tuesday, Mr Ghani noted that "their health has been deteriorating while in the custody of the terrorists".

Anas Haqqani is the son of Jalaluddin Haqqani, founder of the eponymous Taliban affiliate, which is one of the top US targets in the region.

His brother Sirajuddin now leads the Haqqani Network and is deputy head of the Taliban.

Afghan authorities accuse Anas of a being a high-level player in the network. The Taliban has long demanded his release, insisting he is a student.

The other two Taliban prisoners to be released are Haji Mali Khan, believed to be the uncle of Sirajuddin Haqqani, and Abdul Rashid, said to be the brother of Mohammad Nabi Omari, a member of the Taliban's political office in Qatar.

ABC/wires