ISIS militants released a video purporting to show two Japanese hostages on Tuesday and demanded a $200-million ransom.
Global security firm and NBC News counterterrorism consultant Flashpoint Intelligence said the video appeared to be authentic. It features two handcuffed captives in orange garb similar to the jumpsuits worn by Guantanamo Bay detainees kneeling next to a masked jihadi who brandishes a knife.
The militant said they would executed unless $200 million was paid within 72 hours.
On Saturday, Japanese Prime Minister Shinzo Abe pledged about $200 million in non-military assistance for countries battling ISIS.
Abe and other Japanese officials declined to discuss whether they'd pay the ransom. The ISIS video named the hostages as Kenji Goto and Haruna Yukawa. NBC News was unable to confirm their identities.
According to Japanese public broadcaster NHK, Yukawa launched what it described as a "civilian military company" in 2014 and is believed to have traveled to Syria.
Goto has been described as a freelance journalist who has posted reports from Syria on Twitter. His account has not been updated since October 23.
At a news conference in Jerusalem on Tuesday, Abe demanded that the pair be "released immediately."
He added: "Despicable terrorism can never be forgiven ... The $200 million aid which we recently announced, is to be used for people in the region who lost their homes, or to save refugees. It's a humanitarian aid to provide food and medicine."
Yoshihide Suga, Japan's chief Cabinet secretary, described ISIS' threat to kill the hostages as "unforgivable" and said that Tokyo would "cooperate with related nations and do our very best to achieve the release of these Japanese nationals."
U.S. State Department spokeswoman Jen Psaki said: "We stand in solidarity with Japan and are coordinating closely." She called the U.S. calls for the immediate release of the hostages.
ISIS has followed through on its promises to execute captives in the past. On Nov. 16, it released video showing that it had killed American aid worker Abdul-Rahman Kassig after beheading American journalists James Foley and Steven Sotloff earlier in the year.
The group still holds British photojournalist John Cantlie, who has appeared in other extremist propaganda videos, and a 26-year-old American woman captured last year in Syria while working for aid groups. U.S. officials have asked that the woman not be identified out of fears for her safety.
The Associated Press contributed to this report.