Japan has condemned a recording purporting to announce the execution of a Japanese citizen held by Islamic State militants, and demanded the immediate release of another captive depicted as appearing on the image.

"This is an outrageous and unacceptable act of violence," Prime Minister Shinzo Abe told reporters as he arrived at his office after midnight. "We strongly demand the immediate release" of the remaining captive, Kenji Goto.

He later said the government would spare no effort to secure the release of the remaining captive.

Chief Cabinet Secretary Yoshihide Suga condemned what he said was "a video showing what appears to be a murdered Japanese, Mr. Haruna Yukawa, as well as Mr. Kenji Goto."

Relevant government ministers would meet to discuss the situation as the government gathers information, Suga told reporters, declining to take questions.

Reuters could not independently verify the audio message, which if confirmed would be the first time the group, which has beheaded several foreign hostages, has issued an audio message rather than a video showing the actual event.

A deadline by Islamic State militants for Japan to pay a $200 million ransom for Yukawa and Goto expired on Friday. The recording on Saturday purported to show Goto saying Yukawa had been executed and that the militants would release him in exchange for the release of al-Qaeda linked attempted female suicide bomber Sajida Rishwai, an Iraqi held in Jordan.