The Islamic State (IS) militant group has released a video threatening to kill two Japanese hostages unless it receives a $200 million ransom within 72 hours, but Tokyo has vowed it will not give in.

IS has murdered five Western hostages since August last year, but this is the first time the jihadist group — which has seized swathes of Syria and neighbouring Iraq — has threatened Japanese captives.

In the footage, 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 said.

Japanese prime minister Shinzo Abe demanded IS immediately free the two hostages unharmed.

"I am extremely indignant at such an act," he said.

"It is unforgivable and I feel strong resentment against this threat against human life.

"The international community will not give in to terrorism and we have to make sure that we work together."

In the video posted on jihadist websites, a militant said the ransom demand was to compensate for $200 million in non-military aid Mr Abe pledged in support of countries hit by IS violence.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 11 seconds 3 m 11 s Islamic State militants demand $200m ransom for Japanese hostages ( Matthew Carney ) Download 5.9 MB

But Mr Abe said the aid would be disbursed as promised.

"Japan will contribute in non-military fields such as aid for refugees from Iraq and Syria as much as possible, to help refugees who have lost their homes and to provide food and medical service," he said.

"This is unshakeable policy and it will not change."

Japan was not taking part in the US-led air strikes against the Islamic State group but had pledged $200 million to countries that had been hit hard by the militants.

The pledge was part of a bigger $2.5 billion development package the prime minister announced in Cairo last weekend.

Japanese foreign ministry spokesman Ken Okaniwa said the threats would not deter Japan from its pledge.

"There is a strong feeling against such acts, which are unacceptable," he said.

"Japan will not succumb to terrorism. We will continue to contribute to the fight against terrorism.

"We are trying to work closely with other countries in order to secure the release of the hostages. We are making lots of effort."

In a statement, the United States state department said the US was fully supportive of Japan and the two countries were coordinating closely.

Mr Abe was to make the short drive from Jerusalem to visit Palestinian president Mahmud Abbas in the West Bank city of Ramallah before cutting short his six-day tour of the Middle East to concentrate on freeing the hostages.

The Japanese public had feared this kind of situation would be the price for Mr Abe taking a more active role in international security.

Militant in video spoke in English accent

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 southern English accent very similar to the militant who appeared in the footage of the executions of the Britons and Americans.

Mr Abe 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 has spurred an exodus of refugees to neighbouring countries.

One of the hostages, Yukawa, appeared in previous footage posted last August in which he 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 Mr 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 had branch offices in "Turkey, Syria, Africa".

The second hostage, Goto, is a freelance journalist who set up a video production company named Independent Press in Tokyo in 1996.

The company fed video documentaries on the Middle East and other regions to Japanese television networks, including public broadcaster NHK.

AFP