A plan to swap two Western hostages with three Taliban prisoners has been postponed, an Afghan government official told Reuters on Friday, and Taliban sources said the group had moved the Westerners to a "new and safe place".

Afghan president Ashraf Ghani said on Tuesday the government would release a leader of the Taliban's Haqqani militant faction and two other commanders in exchange for two university professors, American Kevin King and Australian Timothy Weeks.

The deal is seen by the Afghan government as a key move in securing direct talks with the Taliban, which has hitherto refused to engage with what it calls an illegitimate "puppet" regime in Kabul.

But a diplomat said in Washington on Wednesday the exchange had not taken place. An Afghan government official told Reuters on Friday it had been postponed, without elaborating further, while Taliban spokesman Zabihullah Mujahid blamed the United States.

“It was a shortcoming from the American side the swap did not happen,” he told Reuters.

Three Taliban sources, including a relative of prisoner Anas Haqqani, brother of the leader of the Haqqani network, said the commanders were due to be flown to Qatar to be freed but were returned to the jail in Bagram outside the Afghan capital Kabul.

"We spoke to them after they were provided with new clothes and shifted out of Bagram jail," the relative said, declining further identification due to the sensitivity of the issue.

"They told us that they were being taken to the plane and we expected them to land in Doha and when it didn't happen for several hours, we got suspicious."

The sources said they had heard about the return of the prisoners to Bagram from Taliban prisoners in the jail and members of the Afghan security forces. The move had left the Taliban "astonished and hurt", said one of the sources, who is familiar with the details of the prisoner exchange.

"The deal was we would free them after our prisoners landed in Qatar," said the third source familiar with the swap.

He said the Taliban had immediately shifted King and Weeks "to a new and safe place" on Tuesday after the commanders failed to land in Doha, home to the Taliban's political leadership.

Taliban sources said they had no information why their prisoners had not been flown to Doha, while spokesmen for the Afghan government and the U.S. embassy in Kabul did not immediately respond to requests for comment.

The Australian government has said on Tuesday it will not provide a "running commentary" on the effort to release Weeks.

The Haqqani network has in recent years carried out large-scale militant attacks on Afghan civilians. It is believed to be based in Pakistan and is part of the Taliban in Afghanistan.

King and Weeks were kidnapped in August 2016 from outside the American University of Afghanistan in Kabul where both worked as professors.

They appeared in a hostage video a year later looking disheveled and pleading U.S. President Donald Trump for their release.