A video released by the Islamic State purports to show the group threatening to kill two Japanese hostages. (Associated Press)
The United States on Tuesday demanded the "immediate release" of two Japanese hostages threatened with death by Islamic State militants.
"The United States strongly condemns ISIL's threat to murder Japanese citizens," State Department spokeswoman Jen Psaki said, referring to the IS group by an alternate acronym.
"We call for the immediate release of these civilians and all other hostages. The United States is fully supportive of Japan in this matter," she said, adding that US Secretary of State John Kerry would speak later in the day with his Japanese counterpart.
The Islamic State group is demanding a $200 million ransom for the return of the two Japanese hostages - freelance journalist Kenji Goto and businessman Haruna Yukawa. Tokyo has vowed not to bow to "terrorism."
In footage posted on jihadist websites, a black-clad militant brandishing a knife addresses the camera in English, standing between the two hostages, who are 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 Japanese Prime Minister Shinzo Abe pledged to support countries affected by Islamic State violence at the start of his Middle East tour.
But Mr Abe, in Jerusalem, has pledged to honor his promise of aid.