Algerian security officials say a search of a desert gas complex raided by Islamists last week has led to the capture of several militants and the discovery of 25 more bodies, indicating the four-day hostage crisis was much deadlier than first thought.

The officials said Algerian special forces detained five militants at the complex of In Amenas in eastern Algeria on Sunday. They said the 25 bodies also discovered at the site likely included hostages who were among the hundreds of Algerians and foreigners working at the facility when the Islamists seized it on Wednesday.

Algerian security forces killed most of the hostage takers in an assault on Saturday. Algeria's state news agency said the militants executed seven hostages during that operation, while Algerian troops killed 11 of the kidnappers. In an earlier report on Saturday, the Algerian government put the official death toll of the siege at 23 hostages and 32 militants, but said the figures were expected to rise.

In a video released Sunday on an Islamist website, militant leader Mokhtar Belmokhtar claimed responsibility for the attack on the gas complex in the name of al-Qaida, saying 40 militants from Muslim and Western nations carried out the raid.

He said his group is ready to “negotiate” with the West if it stops what he called its “bombing of Mali's people” – a reference to French air and ground strikes against al-Qaida-linked rebels in the West African state, which borders Algeria.

Western powers backed the Algerian government's military response to the hostage crisis and its refusal to negotiate with the perpetrators.

British Prime Minister David Cameron said Sunday that responsibility for the killings “lies squarely” with what he called the “terrorists who launched this vicious and cowardly attack.” French President Francois Hollande welcomed what he described as Algeria's “most appropriate” response to “coldly determined terrorists.”

Algerian Communications Minister Mohamed Said said 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 has said an initial rescue operation at the facility freed 107 foreigners and 685 Algerians on Thursday. The foreign hostages included nationals of the United States, Austria, Belgium, Britain, Colombia, France, Japan, Malaysia, Norway, the Philippines and Romania. The complex is jointly run by Algerian, British and Norwegian firms.

British Prime Minister Cameron said 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.

The Japanese government said 10 of its citizens were unaccounted for at the site. It sent Japanese vice foreign minister Minuro Kiuchi to In Amenas on Sunday to find out what happened to them.

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