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A total of 685 Algerian and 107 foreigner workers were freed over the course of the four-day standoff, the Interior Ministry statement said, adding that the group of militants that attacked the remote Saharan natural gas complex consisted of 32 men of various nationalities, including three Algerians and explosives experts. The military also said it had confiscated heavy machine guns, rocket launchers, missiles and grenades attached to suicide belts.

Algeria has fought its own Islamist rebellion since the 1990s, elements of which later declared allegiance to al Qaeda and then set up new groups in the poorly patrolled wastes of the Sahara along the borders of Niger, Mali, Algeria and Libya, where they flourished.

The standoff has put the spotlight on al-Qaeda-linked groups that roam these remote areas, threatening vital infrastructure and energy interests. The militants initially said their operation was intended to stop a French attack on Islamist militants in neighboring Mali — though they later said it was two months in the planning, long before the French intervention.

The militants, who came from a Mali-based al Qaeda splinter group run by an Algerian, attacked the plant Wednesday morning. Armed with heavy machine guns and rocket launchers in four-wheel drive vehicles, they fell on a pair of buses taking foreign workers to the airport. The buses’ military escort drove off the attackers in a blaze of gunfire that sent bullets zinging over the heads of crouching workers. A Briton and an Algerian — probably a security guard — were killed.

The militants then turned to the vast gas complex, divided between the workers’ living quarters and the refinery itself, and seized hostages, the Algerian government said. The gas flowing to the site was cut off.

The accounts of hostages who escaped the standoff showed they faced dangers from both the kidnappers and the military. The militants focused on the foreign workers from the outset, largely leaving alone the hundreds of Algerian workers who were briefly held hostage before being released or escaping.

• Elaine Ganley and Lori Hinnant in Paris contributed to this article.