Japan's Prime Minister says extremist acts by the Islamic State are "unforgivable".
Shinzo Abe's comments come on the heels of a new recording from Japanese hostage Kenji Goto.
In it, the voice, in English, is heard saying a Jordanian pilot currently being held by the militants is going to be killed unless an Iraqi prisoner held in Jordan is freed by sunset on Thursday.
The voice recording is being reviewed by experts to confirm its authenticity.
Japanese Chief Cabinet Secretary Yoshihide Suga says there is a "high probability" the recording is Goto.
"We are currently trying to confirm authenticity of latest audio message. But we think there is a high probability that this is the voice of Mr. Goto, based on various information."
Unconfirmed reports are suggesting the Iraqi woman the Islamic State is demanding has already been released and is now in Syrian territory.
Jordanian authorities have not said whether the reports are true or not.
For more CRI's Xu QinDuo spoke to Ahmed Salah Hashim, Associate Professor of the Military Studies Programme at Nanyang Technological University.
