TOKYO — Prayers were offered Friday at Tokyo’s largest mosque for two Japanese hostages threatened with beheading by Islamic militants who had demanded a $200 million ransom for their release.

Militants affiliated with the Islamic State group posted an online warning that the “countdown has begun” for the extremists to kill 47-year-old Kenji Goto and 42-year-old Haruna Yukawa. The extremists gave Prime Minister Shinzo Abe 72 hours to pay the ransom, and the deadline expired Friday.

The posting, which appeared on a forum popular among Islamic State militants and sympathizers, did not show any images of the hostages, who are believed to be held somewhere in Syria.

The status of efforts to free the men was unclear. Government spokesman Yoshihide Suga was asked about the latest message and said Japan was analyzing it.