Algeria says its special forces ended a siege at a gas complex in the Sahara because Islamist militants were planning to blow it up.
Officials say the entire desert facility has been mined, the BBC reports.
By the end of the operation, the Algerian authorities say the remaining 11 militants at the plant were found dead, along with seven foreign hostages.
Since the crisis began on Thursday, at least 23 hostages and 32 militants have been killed.
A statement from the kidnappers said at the start of their assault on the gas plant that it was launched in retaliation for French intervention against Islamist groups in neighbouring Mali.
United States President, Barack Obama, says the blame for the violent outcome to the siege rested with what he called the terrorists behind the attack.
"We will continue to work closely with all of our partners to combat the scourge of terrorism in the region," said Mr Obama.
His defence secretary, Leon Panetta, says those responsible for all such attacks will be pursued: "We cannot accept attacks against our citizens abroad".
Five Britons are feared dead or missing - five Norwegians are unaccounted for.
French President Francois Hollande defended the Algerian response to the crisis as being "the most suitable".
"When you have people taken hostage in such large numbers by terrorists with such cold determination and ready to kill those hostages - as they did - Algeria has an approach which to me, as I see it, is the most appropriate because there could be no negotiation," he told journalists.
UK Prime Minister David Cameron said that "one British citizen has already been killed in this brutal attack and we now fear the worst for the lives of five others who are not yet accounted for".
"There is no justification for taking innocent life in this way. Our determination is stronger than ever to work with allies right around the world to root out and defeat this terrorist scourge and those who encourage it," Mr Cameron said.
Clearing mines
William Hague: "Our focus is on getting British nationals who have survived this ordeal back to the UK"
The In Amenas gas field is situated at Tigantourine, about 40km south-west of the town of In Amenas and 1300km south-east of Algiers.
The plant is jointly run by BP, Norway's Statoil and Algeria's state-owned oil company.
The militants had been involved in a stand-off since Thursday after trying to occupy the remote site.
Details are still sketchy, but unconfirmed reports say the hostage-takers summarily killed the remaining seven hostages before themselves being killed in a final army raid.
Citing a provisional total from the interior ministry, state news agency APS said 685 Algerian workers and 107 out of 132 foreigners working at the plant had been freed.
At least 23 hostages are known to have died, but the nationalities of some are still not known.
With 14 Japanese nationals thought to be missing, Prime Minister Shinzo Abe said he had received "severe information" about the fate of the hostages.
The chief executive of BP group said 14 of its 18 staff were safe - the company had "grave fears" for the other four.
Helge Lund, chief executive of Norway's Statoil, said the company was still missing five workers and feared "bad news", Reuters news agency reported.
The Algerian interior ministry said troops had recovered:
Algerian national oil and gas company Sonatrach said the army was now clearing mines planted by the militants.
The Algerian interior ministry said troops had recovered:
- six machine guns
- 21 rifles
- two shotguns
- two 60mm mortars with shells
- six 60mm missiles with launchers
- two rocket-propelled grenades with eight rockets
- 10 grenades in explosive belts
Weapons allegedly seized from the kidnappers were shown on Algerian TV.
The crisis began on Wednesday when militants attacked two buses carrying foreign workers. A Briton and an Algerian reportedly died in the incident.
The militants then took Algerians and expatriates hostage at the complex. The leader of the hostage-takers is said to be a veteran fighter from Niger, named as Abdul Rahman al-Nigeri by the Mauritanian news agency ANI, which has been in contact with the militants.
The Algerian armed forces attacked on Thursday as militants tried to move some of their captives from the facility.
