The Islamic State group threatened to kill two Japanese hostages unless it receives a USD 200 million ransom within 72 hours, but Tokyo today vowed it would not give in to "terrorism".
IS has murdered five Western hostages since August last year, but it is the first time that the jihadist group -- which has seized swathes of Syria and neighbouring Iraq -- has threatened Japanese captives.
In footage posted on jihadist websites, a black-clad militant brandishing a knife addresses the camera in English, standing between two hostages wearing orange jumpsuits.
"You now have 72 hours to pressure your government into making a wise decision by paying the USD 200 million to save the lives of your citizens," he says.
The militant says that the ransom demand is to compensate for non-military aid that Prime Minister Shinzo Abe pledged to support countries affected by the campaign against IS during an ongoing Middle East tour that on Tuesday saw him in Jerusalem.
But the Japanese government said it would not bow to extremism.
"Our country's stance -- contributing to the fight against terrorism without giving in -- remains unchanged," chief government spokesman Yoshihide Suga told a news conference in Tokyo.
An official in the foreign ministry's terrorism prevention division had said earlier that the government was investigating the threat and the authenticity of the video.
Since August, IS has murdered three Americans and two Britons, posting grisly video footage of their executions.
US journalists James Foley and Steven Sotloff, American aid worker Peter Kassig and British aid workers Alan Henning and David Haines were all beheaded.
The militant who appeared in the video threatening the Japanese hostages spoke with a very similar southern English accent to the militant who appeared in the footage posted of the executions of the Britons and Americans.
Abe, who was due to give a Jerusalem new conference at 0800 GMT, pledged a total of USD 2.5 billion in humanitarian and development aid for the Middle East on the first leg of his tour in Cairo on Saturday.
He promised USD 200 million in non-military assistance for countries affected by the Islamic State (IS) group's bloody expansion in Iraq and Syria, which spurred an exodus of refugees to neighbouring countries.
The first hostage -- Kenji Goto -- is a freelance journalist who set up a video production company, named Independent Press in Tokyo in 1996, feeding video documentaries on the Middle East and other regions to Japanese television networks, including public broadcaster NHK.
The second hostage appeared in previous footage posted last August in which he identified himself as Haruna Yukawa and was shown being roughly interrogated by his captors.