French President Francois Hollande has come out in support of Algeria's deadly military strike against Islamist hostage-takers at a desert gas plant, saying the action was appropriate in the face of "coldly determined terrorists".

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Other nations have criticised the hasty military backlash that left several expatriate workers dead, with Britain, Japan and Norway insisting they should have been forewarned of an army raid Thursday.

France has refrained from criticising the military action that claimed one of its countrymen among the fallen in the former French colony

"When there is a hostage-taking with so many people involved and such coldly determined terrorists, ready to kill their hostages - which they did - a country such as Algeria has had... the most appropriate responses because there could be no negotiations," Hollande told reporters in Tulle, south-central France.

The captors, calling themselves "Signatories in Blood", killed the last seven of their foreign hostages on Saturday before being gunned down at the remote gas plant, state media said, ending one of the bloodiest international hostage crises in years.

Most of the hostages, including 573 Algerians and about 100 foreigners, had been freed when Algerian forces launched a rescue operation on Thursday, but some 30 remained unaccounted for.

A preliminary government toll Saturday said 23 captives and 32 kidnappers were killed in the four-day hostage drama.

Some analysts say France's non-critical stance to the Algerian events reflects the fraught nature of ties with its former colony, and the fact that the French air force requires access to Algerian airspace for its bombing campaign in neighbouring Mali.

Hollande said the Algerian events justified France's military intervention in Mali, which the hostage-takers had cited as the reason for their action.

"If there had been a need to justify the action that we took against terrorism, we now have an additional argument in favour," the French president said on a visit to Tulle, his political fiefdom, to meet a delegation from an infantry regiment which is deploying troops to Mali.

He added that French troops would stay in the west African state, also a former colony, "as long as is necessary so that terrorism can be defeated in that part of Africa."

Hollande will on Sunday meet the families of seven French hostages being held in the Sahel region.