The posting, which appeared Friday, shows a clock counting down to zero along with gruesome images of other hostages who have been beheaded by the group.
ISIS gave Japanese Prime Minister Shinzo Abe a 72-hour deadline - which expired Friday - to pay a $200 million ransom for the two hostages. The Friday posting on a forum popular among Islamic State militants and sympathizers did not show any images of the Japanese hostages.
In the past, the website has posted Islamic State videos very quickly, sometimes before anyone else. Nippon Television Network first reported the message in Japan.
The status of efforts to free the two men was unclear. Government spokesman Yoshihide Suga, when asked about the latest message, said Japan was analyzing it.
Prime Minister Shinzo Abe convened his National Security Council to discuss how to handle the crisis, as the mother of one of the captives appealed for her son's rescue.
"Time is running out. Please, Japanese government, save my son's life," said Junko Ishido, the mother of 47-year-old journalist Kenji Goto.
"My son is not an enemy of the Islamic State," she said in a tearful appearance in Tokyo, using the name ISIS calls itself.
Ishido said she was astonished and angered to learn from her daughter-in-law that Goto had left less than two weeks after his child was born, in October, to go to Syria to try to rescue the other hostage, 42-year-old Haruna Yukawa.
"My son felt he had to do everything in his power to try to rescue a friend and acquaintance," she said.
In very Japanese fashion, Ishido apologized repeatedly for "all the trouble my son has caused."
The national broadcaster NHK reported early Friday that it had received a message from ISIS "public relations" saying a statement would be released soon.
Lacking clout and diplomatic reach in the Middle East, Japan has scrambled for a way to secure the release of the two men, one a journalist, the other an adventurer fascinated by war.
There was no sign the government had taken action on an offer to try to negotiate with ISIS by Ko Nakata, an expert on Islamic law and former professor at Kyoto's Doshisha University, along with freelance journalist Kousuke Tsuneoka.
Nakata and Tsuneoka, who both are converts to Islam, said Thursday that they have a contact in the Islamic State group and were prepared to go.
Appearing at the Foreign Correspondents Club of Japan, Nakata, who is also a former Islamic specialist at the Japanese Embassy in Saudi Arabia, read a message in Japanese and Arabic.
"Seventy-two hours is just too short. Please wait just a bit longer, and do not try to take action immediately," he said, addressing the militants. "If there is room to talk, I'm ready to go and negotiate."
Nakata proposed offering $200 million in humanitarian aid to refugees and residents of areas controlled by ISIS, through the Red Crescent Society.
Nakata and Tsuneoka, who was released after being held hostage in Afghanistan in 2010, visited Syria in September in an unsuccessful attempt to gain Yukawa's release. Goto was seized sometime after late October when he entered the area.
Ishido said she had not had any contact with the government.
The militants threatened in their video message to kill the hostages unless they received $200 million within 72 hours.
Government spokesman Yoshihide Suga reiterated Friday that Japan was trying all possible channels to reach those holding the hostages, and that its policy of providing humanitarian aid for those displaced by conflict in the Middle East was unchanged.
"We are doing our very best to coordinate with related parties, including through tribal chiefs," Suga said.
Suga confirmed Thursday that the government had confirmed the identity of the two hostages, despite obvious discrepancies in shadows and other details in the ransom video that suggest it may have been altered.
Japanese officials have not directly said whether they are considering paying any ransom, but said their lives were the top priority.
Japan has joined other major industrial nations of the Group of Seven in opposing ransom payments. U.S. and British officials also said they advised against paying.
Tokyo lacks strong diplomatic connections in the Middle East, and Japanese diplomats left Syria as the civil war there escalated, adding to the difficulty of contacting the group holding the hostages.
Since Japan's military operates only in a self-defense capacity at home, any rescue attempt would require help from an ally like the United States.