The planned exchange of two senior Taliban commanders and a leader of the Haqqani militant group for Australian university professor Timothy Weeks has not taken place, a diplomat and a former Afghan official say.

Professor Weeks and his American colleague Kevin King have been held hostage since being kidnapped from the American University of Afghanistan in Kabul in 2016.

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

AAP

The US State Department did not immediately respond to a request for comment, nor did the Afghan and Australian embassies in Washington

"We sincerely appreciate President Ghani's concern for Tim Weeks and Kevin King, and hope the Taliban immediately releases the hostages," a Department of Foreign Affairs and Trade spokesman told AAP on Wednesday.

"The Australian government has never stopped pressing for their release, but we will not give a running commentary on the current process.

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