"This would have been a most-demanding task for security forces anywhere in the world, and we should acknowledge the resolve shown by the Algerians in undertaking it," the British leader said. "The responsibility for these deaths lies squarely with the terrorists."

Such Islamist militant activity is not new to Africa, including recent violence in Mali and Somalia.

Algeria's status as Africa's largest natural gas producer and a major supplier of the product to Europe heightens its importance to those who want to invest there. That interest is coupled with pressure to make sure foreign nationals, and their business ventures, are safe.

Energy and Mining Minister Youcef Yousfi, who a day earlier insisted Algeria can keep its gas facilities secure without foreign forces' help, said he believes the targeted gas facility will be back running "in the shortest possible time" and that foreign workers will soon return. Several foreign companies, including Statoil and BP, evacuated their workers from Algeria after the incident.

"I don't think that these workers have left definitively Algeria," Yousfi told reporters, according to the Algerian Press Service. "Maybe some left ... to reassure their families, but I want to ensure that no company or no worker permanently left the country."

Nations mourn dead, try to account for others

Here is a breakdown on the status of hostages from around the world who were involved in the crisis:

Colombia

Colombia's president said one of its citizens is presumed dead.

France

No known French hostages are unaccounted for, the defense ministry said.

A man identified as Yann Desjeux died after telling French newspaper Sud Ouest that he and 34 other hostages were treated well. It was unclear what led to his death.

Japan

Japanese Vice Foreign Minister Minoru Kiuchi and officials from JGC, a Yokohama-based engineering firm, saw and identified the bodies of seven Japanese citizens killed in the crisis, Chief Cabinet Secretary Yoshihide Suga announced late Monday.

Three Japanese remain unaccounted for, according to Suga.

Malaysia

Three hostages were on their way back home, state media reported. There is a "worrying possibility" that another is dead while a fifth is unaccounted for, the agency said.

Norway

Five Norwegians are missing, while eight are safe, according to Norwegian Prime Minister Jens Stoltenberg.

Philippines