A spokesman said France opens such inquiries when French citizens are suspected to be victims of crimes committed overseas.
There are conflicting reports about the number of people, including expatriate workers, killed when Algerian military forces stormed the In Amenas gas field in eastern Algeria, where militants with links to Al-Qaeda attacked and have kept workers hostage since Wednesday.
In the latest developments:
- From AFP: Algerian Communications Minister Mohamed Said said the assault freed a "large number" of hostages, but news reports from Algiers said nearly 600 of those rescued were Algerian workers.
- The French Prime Minister, Jean-Marc Ayrault, says the military operation is ongoing. Confirms several hostages were killed, but there is no information on the number nor their nationalities.
- An Algerian security source told news agency AFP that 18 militants were killed in the operation on Thursday.
- British energy giant BP, which part operates the plant, said hundreds of workers including 11 of its own staff have been evacuated from Algeria on three flights on Thursday.
- The Algerian news agency APS says two Britons and two Filipinos were killed in the operation on Thursday.
A spokesman for the kidnappers told the Mauritanian news agency ANI 34 captives and 14 hostage takers were killed by Algerian military forces.
But today a security source told the French news agency AFP those numbers were “fantasy”.
Algerian communications Minister Mohamed Said had said late on Thursday that the assault launched by Algerian special forces left "some" of the hostages dead and wounded, but freed a "large number".
The French Interior Minister, Manuel Valls, told radio station RTL two French people escaped the kidnappers and have returned safely.
But he added he did not know about the fate of any other French hostages, only stating there were “a small number” of French expatriates there.
Algiers has come under fire over the lack of concrete information and warning about the oepration.
Britain and Norway, whose state-owned oil firm Statoil is a joint venture partner at the site along with BP and Algeria’s state energy operator, say they were not informed of the operation.
Japan has summoned the Algerian ambassador in Tokyo to demand answers. At least 14 Japanese expatriate workers remain unaccounted for.
France has stopped short of condemning Algeria for the lack of information. The President, François Hollande, said he wished that “the best outcome” would be achived.
Manual Valls also called for “caution with regards to commentary and critiwism” of the operation.
The attackers say they attacked the oil facility because of France’s military intervention to push back Islamist fighters in neighbouring Mali.