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 Islamic State group.
The militant group gave Japanese Prime Minister Shinzo Abe a 72-hour deadline — which expired Friday — to pay a $200 million ransom for the two hostages. The 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 group 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.
"The situation remains severe but we are doing everything we can to win the release of the two Japanese hostages," Suga said. He said Japan is using every channel it can find, including local tribal chiefs, to try to reach the captors.
He said there has been no direct contact from the hostage takers.
At Tokyo's largest mosque, worshippers included the hostages in their Friday prayers.
"All Muslims in Japan, we want the Japanese hostages to be saved as soon as possible," said Sandar Basara, a worker from Turkey.
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.
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.
In very Japanese fashion, Ishido apologized repeatedly for "all the trouble my son has caused." She said she had not had any contact with the government.
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. Japanese diplomats left Syria as the civil war there escalated, adding to the difficulty of contacting the group holding the hostages.
Yasuhide Nakayama, a deputy foreign minister sent to Amman, Jordan, to coordinate efforts to save the hostages, told reporters he had no new information.
"We want to work until the very end, with all our power, to secure their release," he said.
Suga, the government spokesman in Tokyo, said Thursday that the government had confirmed the identity of the two hostages, despite 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. Japan has joined other major industrial nations in the Group of Seven in opposing ransom payments, and U.S. and British officials said they advised against paying.
Two Japanese who said they have contacts with a leader in the Islamic State group offered Thursday to try to negotiate, but it was unclear if the government was receptive to the idea.
Ko Nakata, an expert on Islamic law and former professor at Kyoto's Doshisha University, and freelance journalist Kousuke Tsuneoka are both converts to Islam. They said they have a contact in the Islamic State group and are prepared to go.
Associated Press writers Ashraf Khalil, Kaori Hitomi and Ken Moritsugu contributed to this report.
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