Japan says it is investigating a purported new voice recording of hostage Kenji Goto, who is being held by Islamic State (IS).
The person in the recording says a Jordanian pilot held by IS will be killed unless an Iraqi prisoner held by Jordan is freed by sunset on Thursday.
A spokesman for Prime Minister Shinzo Abe said the recording, posted on YouTube, was being analysed.
Jordan has offered to free the Iraqi prisoner in exchange for the pilot.
On Tuesday, IS published a video where a man claiming to be Kenji Goto said Jordan had 24 hours to free the prisoner.
In the new unverified recording, a man's voice says: "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 time, the Jordanian pilot Moaz al-Kasasbeh will be killed immediately."
Earlier, IS said it had killed another Japanese hostage, Haruna Yukawa.
It had demanded a $200m (£130m) ransom but there was no mention of the ransom in Tuesday's video.
Rishawi is an Iraqi al-Qaeda militant who was sentenced to death in Jordan in relation to bomb attacks in the capital, Amman, in 2005, which killed 60 people.
She was caught with an unexploded suicide bomb belt and confessed to planning an attack with her husband, who was killed.
Moaz al-Kasasbeh was captured on 24 December after his plane crashed in northern Syria.
Jordan is part of the US-led coalition which is carrying out air strikes against IS in Syria and Iraq.
The Jordanian government has said it is ready to release Sajida al-Rishawi, in Jordan, if Lt Kasasbeh is released.
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