In another phone message, al-Nigiri described how half the militants had been killed by the Algerian army on Thursday and that he was ready to blow up the remaining hostages if security forces attacked again.
SITE Intelligence Group, which monitors videos from radicals, posted one showing al-Nigiri with what appears to be an explosive belt strapped around his waist, dating from Jan. 17, after the start of the attack.
Algeria's prisons are filled with militants from the long battle with Islamist extremists that began in the 1990s.
David Plouffe, a senior adviser to President Barack Obama, said Sunday that al-Qaida and al-Qaida-affiliated groups remain a threat in northern Africa and other parts of the world, and that the U.S. is determined to help other countries destroy these networks. Speaking on "Fox News Sunday," Plouffe said the tragedy in Algeria shows once again "that all across the globe countries are threatened by terrorists who will use civilians to try and advance their twisted and sick agenda."
The U.S. State Department issued a travel warning Saturday night for Americans in or traveling to Algeria, citing credible threats of the kidnapping of Western nationals. The department also authorized the departure from Algeria of staff members' families if they choose to leave.
Immediately after the assault, French President Francois Hollande gave his backing to Algeria's tough tactics, saying they were "the most adapted response to the crisis."
"There could be no negotiations" with terrorists, the French media quoted him as saying in the central French city of Tulle.
Hollande said the hostages were "shamefully murdered" by their captors, and he linked the event to France's military operation against al-Qaida-backed rebels in neighboring Mali. "If there was any need to justify our action against terrorism, we would have here, again, an additional argument," he said.
On Sunday, French Foreign Minister Laurent Fabius said he was "appalled" at the idea that blame would be laid on Algerian authorities instead of the jihadist captors.
"The terrorists . . . they're the ones to blame," Fabius said on France's iTele TV channel. He said Algerian officials were in touch with the French during the crisis. "But they didn't have to tell us: `Here is what we will do.' "
In the final assault, the remaining band of militants killed seven hostages before 11 of them were in turn cut down by the special forces, Algeria's state news agency said. The military launched its Saturday assault to prevent a fire started by the extremists from engulfing the complex and blowing it up, the report added.
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 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-Qaida 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-Qaida-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-Qaida 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.