Foreign governments are urgently seeking information about the fate of their citizens caught up in the siege at a desert gas facility in Algeria.
British, US, Norwegian, and Japanese nationals are still missing after the four-day siege, which was ended by an Algerian army raid on Saturday.
US President Barack Obama has blamed "terrorists" for the deaths of at least 23 hostages at In Amenas.
Algeria said its troops had killed all 32 hostage-takers.
Officials said the army launched its assault after Islamist militants began killing foreign hostages.
Missing
Three Britons are confirmed dead, and a further three are missing, feared dead. Their families had undergone an "absolutely dreadful ordeal", UK Prime David Cameron said on Sunday.
UK officials were "working hard" to locate the missing, said Foreign Secretary William Hague.
Colombia has confirmed the death of one of its citizens, Carlos Estrada, who was a British resident and worked for BP.
Japanese Prime Minister Shinzo Abe warned of bad news to come.
"According to what we have heard from the Algerian government, there is indeed grim information about our citizens," he said.
Mr Abe said he had asked the Algerian prime minister to make all efforts towards confirming the status of Japanese nationals.
Two Malaysians are unaccounted for, as are five Norwegians.
"This attack is another reminder of the threat posed by al-Qaeda and other violent extremist groups in North Africa," said Mr Obama on Saturday.
"We will continue to work closely with all of our partners to combat the scourge of terrorism in the region."
Mortars and RPGs
State news agency APS said 685 Algerian workers and 107 out of 132 foreigners working at the plant had been freed, citing interior ministry figures.
The nationalities of some of the hostages killed are still not known.
The UK Foreign Office has confirmed that some British nationals caught up in the Algerian hostage crisis returned home overnight.
No details were released, but a spokesman said the foreign secretary would give an update later on Sunday.
The crisis began on Wednesday when militants attacked two buses carrying foreign workers the remote site in south-eastern Algeria. A Briton and an Algerian reportedly died in the incident.
The militants then took Algerians and expatriates hostage at the complex, which was quickly surrounded by the Algerian army.
A statement from the kidnappers said the assault on the gas plant was launched in retaliation for French intervention against Islamist groups in neighbouring Mali.
However, France only decided last week to intervene militarily in Mali. Analysts say the assault on the gas facility was well-planned and would have required advance research, as well as possibly inside help.
The leader of the hostage-takers was a veteran fighter from Niger, named as Abdul Rahman al-Nigeri by the Mauritanian news agency ANI, which had 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.
'Suitable' response
APS reported before Saturday's second raid that a group of militants remained holed up in a workshop with the remaining hostages.
After the raid, 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
One Algerian worker, who gave his name only as Chabane, told the Associated Press that at one point the militants caught a man he described as a Briton.
"They threatened him until he called out in English to his friends, telling them, 'Come out, come out, they're not going to kill you. They're looking for the Americans'," Chabane told AP.
"A few minutes later, they blew him away," he said. Chabane's account could not be independently confirmed.
French President Francois Hollande defended the Algerian response to the crisis as being "the most suitable".
UK Prime Minister David Cameron said there was "no justification" for the hostage-taking.
"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," said Mr Cameron.
The In Amenas gas field is situated at Tigantourine, about 40km (25 miles) south-west of the town of In Amenas and 1,300km (800 miles) south-east of Algiers.
The plant is jointly run by BP, Norway's Statoil and Algeria's state-owned oil company.
- Bus attack: 0500 local time 16 January: Heavily armed gunmen attack two buses carrying gas field workers towards In Amenas airfield. A Briton and an Algerian die in the fighting.
- Hostages taken: The militants drive to the installation at Tigantourine and take Algerian and foreign workers hostage in the living area and the main gas facility at the complex.
- Army surround complex: Security forces and the Algerian army surround the hostage-takers. Western leaders, including the UK's David Cameron, urge Algeria to consult them before taking action.
- Army attacks: 1200 (1300 GMT) 17 January: Algerian forces attack as militants try to move some of their captives from the facility. Reports say some hostages escape, but others are killed.
- Final assault: The Algerians ended the raid on 19 January, killing the last 11 captors after they had killed seven hostages, state media reported. Twenty-three hostages and 32 militants in total are now known to have died.