Japan promised not to give up "until the very end" on efforts to rescue two Japanese hostages threatened with beheading by Islamic militants, after a deadline passed with no word from the captors.

Militants affiliated with the Islamic State (IS) group posted an online warning Friday that the "countdown has begun" for the extremists to kill 47-year-old Kenji Goto and 42-year-old Haruna Yukawa. The extremists had given Japanese prime minister Shinzo Abe 72 hours to pay the £133 million ransom. The posting, which appeared on a forum popular among IS militants and sympathisers, did not show any images of the hostages, who are believed to be held somewhere in Syria.