Islamic State militants have extended the deadline for freeing hostages to Thursday evening, saying that the Japanese journalist and Jordanian pilot would be killed unless an Iraqi female prisoner was released by sunset.
The audio message believed to be in the voice of Kenji Goto was posted on YouTube early on Thursday.
"I am Kenji Goto. This is a voice message I've been told to send to you. If Sajida al-Rishawi is not ready for exchange for my life at the Turkish border by Thursday sunset 29th of January Mosul (Iraq) time, the Jordanian pilot Muath al-Kasaesbeh will be killed immediately," the voice in the recording says.
The audio message follows a video released on Tuesday showing Goto saying he had 24 hours to live unless Jordan released al-Rishawi, the Iraqi woman on death row in Jordan for her role in a 2005 suicide bomb attack that killed 60 people in the capital Amman.
Jordan has received no assurance so far that al-Kasaesbeh was safe and indicated that it would go ahead with a proposed prisoner swap only if he was freed, reports Reuters.
The audio tape message, however, does not indicate that the Jordanian pilot would be part of the exchange deal.
Sources said Jordanian authorities were trying to verify the authenticity of the recording and were coordinating with their Japanese counterparts.
Kasaesbeh was captured after his jet crashed in north-eastern Syria in December during a bombing mission against Islamic State.
Speaking in parliament on Thursday, Japanese Prime Minister Shinzo Abe said the government was making every effort to ensure Goto's early release and repeated that Japan was seeking cooperation from Jordan.
He reiterated that Japan would not give in to terrorism.
Japanese journalist Goto went to Syria in late October in search of his missing friend Haruna Yukawa, who was captured and recently beheaded by the militants.