TOKYO — Militants affiliated with the Islamic State group have posted an online warning that the “countdown has begun” for the group to kill a pair of Japanese hostages.

The posting which appeared today 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 today — 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. 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 there has been no direct contact from the hostage takers.

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.