Algerian officials say security forces searching a desert gas complex raided by Islamists have found the bodies of more hostages killed by the militants in the four-day seizure of the facility this week.

Security officials said Sunday an additional 25 bodies were discovered at the complex at Ain Amenas in eastern Algeria. They said the bodies appeared to be those of hostages who were among the hundreds of Algerians and foreigners working at the facility when it was seized by the Islamists on Wednesday. Algerian security forces ousted the militants from the complex in a deadly assault on Saturday.

Algeria's state news agency said the militants killed seven hostages during that operation, while Algerian troops killed 11 of the hostage-takers. In an earlier report on Saturday, the Algerian government said the overall death toll from the gas complex siege stood at 23 hostages and 32 militants.

British Prime Minister David Cameron said Sunday that three British hostages were confirmed dead, and another three were believed to have been killed. U.S. officials have confirmed the death of one American at the site.

Mr. Cameron said responsibility for the killings “lies squarely” with what he called the “terrorists who launched this vicious and cowardly attack.” French President Francois Hollande endorsed Algeria's handling of the situation, saying it was the “most appropriate” response to “coldly determined terrorists.”

Algerian Communications Minister Mohamed Said said Sunday the Islamist assailants came from six nations. He said they had laid mines around the gas complex and security forces were trying to clear them.

Algeria said it had freed 107 foreign hostages and 685 Algerians in an initial assault on the facility on Thursday.

The foreign hostages included nationals from the United States, Britain, Japan, Norway, Romania, the Philippines, France, Malaysia and Austria. The complex is jointly run by Algerian, British and Norwegian firms. Japanese officials have said several of their nationals are missing.

The U.S. State Department has issued a travel advisory to Americans in Algeria, warning of credible threats of additional kidnappings of Western nationals.

The militants said they carried out the attack in retaliation for French military operations in Mali.