The posting, which appeared Friday, also shows a clock counting down to zero along with gruesome images of other hostages who have been beheaded by the group, according to the Associated Press.
The website it was posted on has at times been among the first to host videos released by Isis, AP reports. It said Nippon Television Network first reported the message in Japan.
The post reportedly did not include any images of the Japanese hostages. The Independent is trying to verify this.
A video emerged on Tuesday showing a black-clad militant wielding a knife and threatening to execute Kenji Goto and Haruna Yukawa if the Japanese Government did not pay a $200 million (£132m) ransom within 72 hours.
Tokyo believes the deadline expired at 5.50am GMT on Friday, according to AFP.
Junko Ishido, the mother of 47-year-old journalist Goto, made a tearful appeal on behalf of her son as the ransom deadline demanded by Isis for Mr Goto and Mr Yukawa, drew closer.