Japan is grieving over its greatest loss of life at militant hands since 9/11 as the carnage in Algeria provoked soul-searching for a people who have been relatively immune to Islamist terror.

There was blanket media coverage of the news that at least seven Japanese nationals had been killed in the Algerian hostage crisis, with the respected business daily Nikkei describing Japan's anger as "overwhelming".

Three other Japanese remained unaccounted for, after heavily armed Islamist gunmen overran the desert gas complex for four days before most were killed by Algerian security forces. At least 37 foreign hostages died in all.

A government plane was to leave Japan later bound for Algeria. It was expected to return on Thursday with survivors and the bodies of those killed, all of whom were employees or contractors for Japanese engineering firm JGC.