Experts are examining a recorded message said to have been from hostage Kenji Goto, extending the deadline for Jordan's prisoner-swap release of an Iraqi would-be suicide bomber linked to al Qaida.
The message, read by a voice claiming to be Japanese hostage Kenji Goto, was released online after Jordan offered the prisoner exchange to Islamic State (IS) militants, desperately seeking to save a Jordanian air force pilot they have threatened to kill, along with Mr Goto.
The recording, in English, says the Jordanians must present Sajida al-Rishawi at the Turkish border by sunset today, or Jordanian pilot Lt Mu'ath al-Kasaseabeh will be killed.
The recording, distributed on Twitter by IS-affiliated accounts, could not be verified independently.
In Tokyo, Japanese government spokesman Yoshihide Suga said the government was analysing the latest message and Japan was doing its utmost to secure the release of Mr Goto, working with nations in the region including Turkey, Jordan and Israel.
"We are trying to confirm (the message), but we think there is a high probability that this is Mr Goto's voice," he said.
Mr Suga would not comment on the specifics of the talks with Jordan, saying the situation was developing. The cabinet is meeting to assess the latest developments.
In comments in parliament, prime minister Shinzo Abe repeated his condemnation of the IS hostage-taking, saying: "This heinous terrorist act is totally unforgivable."
Releasing the would-be hotel bomber would breach Jordan's usual hard-line approach to the extremists, setting a precedent for negotiating with them.
IS has not publicly demanded prisoner releases before and Jordan's main ally, the United States, opposes negotiations with extremists.
But King Abdullah faces growing domestic pressure to bring the pilot home. Mr Kasaseabeh's father said he met the king yesterday, who assured him that "everything will be fine".
Efforts to free Mr Kaseasbeh and Mr Goto gained urgency after a purported online ultimatum claimed on Tuesday that IS would kill both hostages within 24 hours if Jordan did not free Rishawi.
The scope of a possible swap and of IS's demands remained unclear.
Jordanian government spokesman Mohammed al-Momani said Jordan was ready to trade the prisoner, an Iraqi woman convicted of involvement in deadly Amman hotel bombings in 2005, for the pilot. He made no mention of Mr Goto.
Rishawi's release would be a coup for the extremists, who have 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 IS targets in Syria and Iraq in recent months.
The pilot's capture has hardened popular opposition to the campaign in Jordan, analysts said.
"Public opinion in Jordan is putting huge pressure on the government to negotiate with the Islamic State group," said Marwan Shehadeh, a scholar with ties to ultra-conservative Islamic groups in Jordan.
"If the government doesn't make a serious effort to release him, the morale of the entire military will deteriorate and the public will lose trust in the political regime."
The pilot's family has criticised the government and several dozen protesters including his father gathered outside King Abdullah's palace in Amman.
His father Safi Kasaesbeh was allowed into the palace, along with his wife, to meet Abdullah.
"The king told me that Muath is like my son and God willing everything will be fine," he said afterwards.
Jordan reportedly is holding indirect talks with the militants through religious and tribal leaders in Iraq to secure the release of the hostages.
In his brief statement, Mr Momani said only Jordan was willing to swap Rishawi for the pilot, but not if such an exchange was being arranged. Rishawi was sentenced to death for her involvement in the al Qaida attack in Amman that killed 60 people.
In Tokyo, Mr Goto's mother, Junko Ishido, has been appealing to prime minister Shinzo Abe to keep trying to save her son. "Kenji has only a little time left," she said.
The militants have reportedly killed another Japanese hostage, Haruna Yukawa, and the crisis has stunned Japan.
Mr Kasaseabeh, 26, was seized after his Jordanian F-16 crashed in December near the 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.
Mr Goto, a freelance journalist, was captured in October in Syria, apparently while trying to rescue Yukawa, 42, who was taken hostage last summer.
IS broke with al Qaida's central leadership in 2013 and has clashed with its Syrian branch, but it reveres the global terror network's former Iraqi affiliate, which battled US forces and said it carried out the 2005 Amman attack.