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.