People walk past television screens displaying a news program, about an Islamic State video showing Japanese captive Kenji Goto, on a street in Tokyo January 28, 2015. [Photo/Agencies]

The audio tape message implied that the Jordanian pilot would not be part of the exchange deal, indicating any swap would be between Goto - a veteran war reporter - and al-Rishawi. Any swap that left out the pilot would not go down well with the public in Jordan, where officials have insisted he is their priority.

There was no immediate comment from Jordanian government officials, but a security official said the authorities were trying to verify the authenticity of the recording and were coordinating with their Japanese counterparts.

On Tuesday, a video was released purporting to show the Japanese national saying he had 24 hours to live unless Jordan released al-Rishawi, an Iraqi woman on death row for her role in a 2005 suicide bomb attack that killed 60 people in the capital Amman.

Jordanian government spokesman Mohammad al-Momani said earlier that Jordan was ready to release al-Rishawi if Kasaesbeh was spared, but made clear that she would be held until the pilot was freed.

Kasaesbeh was captured after his jet crashed in northeastern Syria in December during a bombing mission against Islamic State, which has captured large tracts of Syria and Iraq. He is from an important Jordanian tribe that forms the backbone of support for the Hashemite monarchy.