Troops found the bodies of 25 captives during the raid of the isolated plant located in the Sahara Desert.
Algerian spokesman Mohamed Said said the death toll could rise, as reported by USA Today.
"I am very concerned that this preliminary death toll will be, unfortunately, revised upwards in the coming hours," he reportedly said.
Militants attacked the plant on January 16 after attacking a bus with facilities workers on the way to the airport.
Algerian forces began the assault on the facility on Thursday, much to the surprise of Western nations which were not consulted on the move.
French President Francois Holland said Algeria's tactics were the "most adapted response" and there could be no negotiations with terrorists, as reported by the Financial Times.
He linked the hostage situation to France's military operation in neighbouring Mali against al-Qaeda backed rebels.
"If there was any need to justify our action against terrorism, we would have here, again, an additional argument."
In a statement US President Barack Obama said he would remain in contact with the Algerian government and the US would provide the Algerian government with assistance in dealing with the attack.
"This attack is another reminder of the threat posed by al-Qaeda and other violent extremist groups in north Africa," Obama said.
"In the coming days, we will remain in close touch with the government of Algeria to gain a fuller understanding of what took place so that we can work together to prevent tragedies like this in the future."
Statoil said the situation was "unresolved" for five of its employees, while BP provided no information on the status of four missing employees.