Jordan has demanded proof of life of a military pilot held by Islamic State militants before moving ahead with any possible swap to bring about his release.
The militants have apparently threatened to kill the air force pilot, Lieutenant Muath al-Kaseasbeh, by sunset Iraqi time unless Jordan frees an Iraqi woman from death row and delivers her to the Turkish border.
The woman, Sajida al-Rishawi, was involved in deadly Amman hotel bombings a decade ago.
Jordanian government spokesman Mohammed al-Momani said al-Rishawi is still in Jordan, raising doubts that the sunset deadline can be met.
He said: "We want to see a proof of life of the Jordanian pilot and then we can talk about the exchange."
An IS audio message, read in English by a voice the Japanese government says is probably that of another hostage, Kenji Goto, was released last night after Jordan offered to hand over al-Rishawi in exchange for Lt al-Kaseasbeh.
The recording says Jordan must present al-Rishawi at the Turkish border tonight in exchange for Mr Goto's life or the pilot would be killed. It was not clear what Mr Goto's fate would be if the woman was not returned.
The message said the deadline was sundown in Mosul, Iraq, the largest city held by the militant group.
In Tokyo, government spokesman Yoshihide Suga said officials were in close communications with the Jordanian government. He said Japan was doing its utmost to free Mr Goto, working with nations in the region including Turkey, Jordan and Israel.
"As the situation is developing, I shouldn't comment on details. But, Japan and Jordan are dealing with the matter based on an extremely trusting relationship," Mr Suga told reporters.
Efforts to free Lt al-Kaseasbeh and Mr Goto gained urgency after an apparent online ultimatum on Tuesday claimed that IS would kill both hostages within 24 hours if Jordan did not free al-Rishawi.
The crisis began last week with the release of a video by IS showing Mr Goto and another Japanese hostage, Haruna Yukawa, kneeling in orange jumpsuits with a masked man who threatened to kill them within 72 hours unless Japan paid a $200 million ransom.
That demand has since shifted to one for the release of al-Rishawi. The militants have reportedly killed Mr Yukawa, 42, although that has not been confirmed.
"This heinous terrorist act is totally unforgivable," prime minister Shinzo Abe said in parliament today.
Mr Goto, a freelance journalist, was captured in October in Syria, apparently while trying to rescue Mr Yukawa, who was taken hostage last summer.
In Tokyo, Mr Goto's mother Junko Ishido has been desperately pleading for the government to save her son.
"I know Mr Abe is someone who can handle this matter. I trust Mr Abe and I can do nothing but rely on him," she said.
Releasing the would-be hotel bomber linked to al Qaida would breach Jordan's usual hardline approach to the extremists and set a precedent for negotiating with them.
It would also be a coup for IS, which has already overrun large parts of neighbouring Syria and Iraq. Jordan is part of a US-led military alliance that has carried out air strikes against the extremist group in Syria and Iraq in recent months.
IS has not publicly demanded prisoner releases before and Jordan's main ally, the United States, opposes negotiations with extremists.
Jordanian King Abdullah II faces growing domestic pressure to bring the pilot home. The pilot's father said he had met Jordan's king, who he said assured him that "everything will be fine".
Jordan is reportedly holding indirect talks with the militants through religious and tribal leaders in Iraq to secure the release of the hostages.
The 26-year-old pilot was seized after his Jordanian F-16 crashed in December near IS's de facto capital of Raqqa in Syria. He is the first foreign military pilot the militants have captured since the coalition began its air strikes in August.
Previous captives may have been freed in exchange for ransom, although the governments involved have refused to confirm any payments were made.