The Islamic State (IS) group threatened to kill two Japanese hostages unless it receives a $200 million ransom within 72 hours, in a video released Tuesday, but Tokyo has vowed it will not give in to "terrorism".
IS has murdered five Western hostages since August last year, but it is the first time the militant group which has seized swathes of Syria and neighbouring Iraq -- has threatened Japanese captives.
In footage posted online, 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 $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 in Mideast -
Abe, who was due to give a Jerusalem news conference at 0800 GMT, pledged a total of $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 $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.
He was born in Sendai, Miyagi, in 1967, according to the company's website.
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.
Another online video that appeared at the time showed a man believed to be Yukawa test-firing an AK-47 assault rifle in Syria.
The same video could be seen on the website of Tokyo-based private military firm PMC, which listed Yukawa as its chief executive.
Calls to the firm at the time went unanswered and it was unclear if the company had other employees. Its website said the firm has branch offices in "Turkey, Syria, Africa".
Japanese nationals' involvement as combatants in foreign conflicts is limited, although the country's extensive media is usually well-represented in hotspots.
Japan has been relatively isolated from attacks that have hit other developed countries, having tended to stay away from US-led military interventions.
The country was rocked in early 2013 when militants overran a remote gas plant in the Algerian desert. The four-day ordeal that involved hundreds of hostages ended when Algerian commandos stormed the plant.
Ten Japanese died, giving the country the single biggest body count.
The hostage-takers said they had launched the raid in response to military action against militants in Mali.
In response, Tokyo pledged $120 million in fresh aid to help stabilise the Sahel region, which runs across North Africa.