The militants threatened in a video message to kill the hostages within 72 hours unless they receive $200m (£132m). Based on the video’s release time, that deadline will expire sometime today.
Government spokesman Yoshihide Suga said that Japan was trying all possible ways to reach those holding the hostages – 47-year-old freelance journalist Kenji Goto, and 42-year-old Haruna Yukawa, the founder of a private security company.
Japan has not received any message from IS since the release of the video, he said.
The crisis is a test of Prime Minister Shinzo Abe’s push to expand Japan’s role in international affairs and raise the profile of its military.
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.
So far, the only initiative made public was an offer by Ko Nakata, an expert on Islamic law and former professor at Kyoto’s Doshisha University.
Appearing at the Foreign Correspondents’ Club of Japan, Mr Nakata, who is also a former Islamic specialist at the Japanese Embassy in Saudi Arabia, read a message in Japanese and Arabic.
“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.”