Isis has issued a video threatening to kill two Japanese hostages, identified as Kenji Goto Jogo and Haruna Yukawa, unless their government paid a $200m (£133m) ransom within 72 hours, according to a Sky News report.
In the video, a black-clad militant brandishing a knife addresses the camera in English as he stands between two hostages wearing orange jumpsuits.
“You now have 72 hours to pressure your government into making a wise decision by paying the $200m to save the lives of your citizens,” he says.
The footage, identified as being made by the militant group’s al Furqan media arm and posted on militant websites, also sees the militant criticising the Japanese government’s support for US-led airstrikes against IS.
The Japanese government have however refused to give in to their ransom demand.
Prime Minister Shinzo Abe told a news conference in Jerusalem: “I strongly demand they not be harmed and that they be immediately released.
“I am extremely indignant at such an act.”
The militant who appears in the video is apparently the same man involved in the beheadings of other IS captives.
He says: “To the prime minister of Japan: Although you are more than 8,000 and 500 kilometres (5,280 miles) from the Islamic State, you willingly have volunteered to take part in this crusade.
“You have proudly donated $100m to kill our women and children, to destroy the homes of the Muslims.”
In August, a Japanese citizen believed to be Mr Yukawa – a private military company operator – was kidnapped in Syria.
Mr Goto is a freelance journalist who went to report on Syria’s civil war last year.
The Japanese Prime Minister is currently on a six-day visit to the Middle East.
Last week, he pledged around $200m in non-military assistance for countries battling Isis.
Japan’s foreign ministry said it was checking the video, but if it was genuine “such a threat by taking hostages is unacceptable and we are extremely resentful”.