BEIRUT (AFP) - The Islamic State in Iraq and Syria (ISIS) threatened to kill two Japanese hostages unless Tokyo pays a US$200 million (S$266 million) ransom within 72 hours, in a video posted on militant websites on Tuesday.

In the video, 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 dollars to save the lives of your citizens," he says.

The militant says that the ransom demand was to compensate for non-military aid that Prime Minister Shinzo Abe pledged to support the campaign against ISIS during an ongoing Middle East tour that on Tuesday saw him in Jerusalem.

The Japanese government said on Tuesday it will not give in to "terrorism". "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.

Earlier, Tokyo had said that it was looking into the threat. "We are aware of the reports. We are in discussions on the matter," said an official in the Foreign Ministry's terrorism prevention division, declining to be named.

When asked whether the government regarded the video as authentic, he told AFP: "We are checking that too."

One of the hostages appeared in previous footage posted last August in which he identified himself as Haruna Yukawa and was shown being roughly interrogated by his captors.

The second 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, Miyagki, in 1967, according to the company's website.