It had initially been unclear whether the bodies found were those of hostage-takers or staff at the facility.
A search is continuing at the In Amenas gas plant, where as many as 20 hostages remain unaccounted for.
Five suspected Islamist attackers were reportedly arrested on Sunday.
The Algerian authorities had said on Saturday that all 32 hostage-takers had been killed. The suspected organiser of the attack, Mokhtar Belmokhtar, said 40 militants had taken part.
The siege ended when troops mounted an assault on Saturday, saying the militants had begun killing foreign hostages.
Western leaders condemned the attack with French Defence Minister Jean-Yves Le Drian describing it as an "act of war".
The plant, which is responsible for more than a tenth of Algeria's overall gas output, is expected to resume production by Tuesday, Algeria's energy minister said.
Prime Minister Abdelmalek Sellal is due to give a new conference about the attack later on Monday.
Youcef Yousfi also pledged to boost security at energy installations but insisted it was "out of the question to allow foreign security forces to handle the security" of Algerian oil facilities.
During a visit to the affected plant on Sunday, Mr Yousfi said it would resume production within two days.
Security forces reportedly discovered the bodies of 25 hostages as they searched the complex for booby-traps and mines.
The militants had threatened to blow up the site and kill their hostages, officials said.
Belmokhtar, who is not thought to have been among the actual attackers, said his group had carried out the attack. He was speaking in a video message carried by the Mauritanian website Sahara Media.
The website said the video had been recorded on 17 January while the siege was still going on but not posted on the website.
It shows Belmokhtar, who has convictions in absentia for murder, kidnapping and terrorism, saying he was prepared to negotiate with Western and Algerian leaders if operations against Islamists in Mali were stopped.