An audio message purportedly from a Japanese journalist being held by Islamic State militants said a Jordanian air force pilot also held by the group would be killed unless an Iraqi female prisoner in Jordan was released by sunset on Thursday.

The message appeared to postpone a previous deadline set on Tuesday in which the journalist, Kenji Goto, said he would be killed within 24 hours if the Iraqi was not freed.

The latest audio recording, which could not be verified by Reuters, was posted on YouTube early on Thursday. The Japanese government is analyzing the recording, Prime Minister Shinzo Abe told a panel in parliament.

"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.

Jordan said on Wednesday it had received no assurance that al-Kasaesbeh was safe and that it would go ahead with a proposed prisoner swap only if he was freed.

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.

Test for Abe

The Jordanian comments have raised concerns in Japan that Goto might no longer be part of any deal between Amman and Islamic State. But CNN quoted Jordanian Foreign Minister Nasser Judeh as saying that "of course" the Japanese hostage's release would be part of any exchange.

The hostage crisis is the biggest diplomatic test for Abe since he took office in 2012 pledging to bolster Japan's defence and play a bigger role in global security.

Speaking in parliament on Thursday, Abe said the government was making every effort to ensure Goto's early release and repeated that Japan was seeking cooperation fromJordan.

He reiterated that Japan would not give in to terrorism and that Tokyo would keep cooperating with the international community.

"If we are too afraid of terrorism and give in to it, this will give rise to fresh terrorism against Japanese and it will become a world in which the will to carry out despicable violence has its own way," Abe told a lower house budget committee. "Such a thing is totally impermissible."

The crisis erupted after Abe, while on a tour of the Middle East, announced $200 million in non-military aid for countries contending with Islamic State, but his government has rejected any suggestion it acted rashly and stressed the assistance was humanitarian.

Abe added that peace and stability of the Middle East were important for Japan's energy strategy. Resource-poor Japan relies heavily on the Middle East for oil imports.

Goto went to Syria in late October. According to friends and business associates, he was attempting to secure the release of Haruna Yukawa, his friend and fellow Japanese citizen who was captured by Islamic State in August.

In the first of three videos purportedly of Goto, released last week, a black-clad masked figure with a knife said Goto and Yukawa would be killed within 72 hours if Japan did not pay Islamic State $200 million.

A video on Saturday appeared to show Goto with a picture of a decapitated Yukawa, saying his captors' demands had switched to the release of al-Rishawi.

Tuesday's video featured an audio track over a still picture that appeared to show Goto holding a picture of a now bearded Kasaesbeh.