Algerian Prime Minister Abdelmalek Sellal has said the 32 militants who took dozens of people hostage at In Amenas gas plant had “come from northern Mali”.
As many as 48 hostages – including foreigners – are thought to have died at the site near the town of In Amenas.
About 20 captives remain unaccounted for after the four-day siege, which ended on Sunday.
The militants said they took hostages in retaliation for French intervention against Islamists in Mali.
Algerian Prime Minister Abdelmalek Sellal has said the 32 militants who took dozens of people hostage at In Amenas gas plant had come from northern Mali
Five suspected members of the Islamist group which held foreign and local workers hostage at In Amenas gas plant in Algeria have been arrested, reports say.
The reports came a day after the Algerian authorities said all 32 hostage-takers had been killed at the In Amenas gas installation.
At least 25 bodies were found at the complex on Sunday, reports say.
It is unclear whether they were captors or captives. Officials say a definitive death toll will be released later.
On Saturday officials said least 23 staff at the facility had died during the four-day siege, with some Western workers still unaccounted for.
The siege was ended in a raid by troops on Saturday.
Officials said the army launched its assault after Islamist militants began killing foreign hostages.
UK Prime Minister David Cameron and US President Barack Obama have blamed “terrorists” for the hostages’ deaths.
Five suspected members of the Islamist group which held foreign and local workers hostage at In Amenas gas plant in Algeria have been arrested
And on Sunday French Defence Minister Jean-Yves Le Drian described the hostage-taking as an “act of war”.
“What strikes me the most is that we’re saying <<hostage-taking>> but when there are so many people concerned, I think this is an act of war,” he told French TV.
“Five terrorists were found still alive this morning,” said the private Ennahar TV channel, quoted by AFP news agency.
The agency said residents of the nearby town of In Amenas were staying indoors, amid rumors that the army operation to end the siege was not over.
Algerian Communications Minister Mohammed Said said earlier that the militants were from six different countries, “nationals of Arab and African countries, and of non-African countries”.
Mohammed Said added that a final death toll would be released in the coming hours.
Mauritanian website Sahara Media says Mokhtar Belmokhtar, the suspected organizer of the siege, has claimed responsibility for it in a video message.
The website said the video – recorded on January 17 while the siege was still going on but not posted on the website – showed the militant leader saying he was prepared to negotiate with Western and Algerian leaders if operations against Islamists in Mali were stopped.
Three Britons are confirmed dead, and a further three are missing, feared dead.
UK officials were “working hard” to locate the missing, said Foreign Secretary William Hague.
“Everything seems to indicate” that a Colombian citizen resident in the UK is among the dead, the Colombian president has said.
But he added that information about Carlos Estrada, who worked for BP, was “not 100%”.
Japanese officials said they had no confirmation of the fate of 10 nationals who remained unaccounted for, despite reports that nine had died.
Foreign Ministry spokesman Yoshihide Suga said a government aircraft would be sent to bring home seven others who had survived.
Two Malaysians are unaccounted for, as are five Norwegians.
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 crisis began on Wednesday when militants attacked two buses carrying foreign workers to 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 neighboring 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.