"There were very few French on this base, which was particularly sprawling" and remote. "Maybe that's why it's difficult for information to get back to us," Valls told RTL radio.
"We have news from two of those who are back. Regarding the two others, if there were two others, we don't have more information at this stage and hope to have more later in the morning," he added.
Valls said it was also unclear how many people had died in the assault on the plant because of the "confused" situation on the ground. He was cautious when asked for a reaction to the Algerian military operation against the site.
"When we are confronted with terrorism, when we fight together, we should be careful with criticisms," Valls said, adding that he was not aware of all the conditions surrounding the operation.
"We don't know if the operation is over, so I am staying extremely careful about the analysis that must be done of this situation," he said. Asked about media reports that one of the hostage-takers was a French citizen, Valls said: "We have no information."
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A British Foreign Office spokeswoman said Friday that the "terrorist incident in Algeria remains ongoing."