* Hostage toll to rise from provisional 23 - minister
* At least three Britons dead - Cameron
* France cautions against criticising Algerian response
* Military clearing plant of mines after "final assault"
By Lamine Chikhi and Abdelaziz Boumzar
ALGIERS/IN AMENAS, Algeria, Jan 20 (Reuters) - Algeria warned on Sunday that the hostage death toll from a siege at a desert gas plant would rise, after its troops staged a final assault which killed all the remaining Islamist gunmen.
Britain said at least three of its citizens had been killed in the crisis, ended when Algerian special forces stormed the In Amenas plant on Saturday. France acknowledged the death toll would be high but cautioned against criticising Algeria's military response, saying it had faced an intolerable situation.
Algeria's Interior Ministry had reported on Saturday that 23 hostages and 32 militants were killed during assaults launched by Algerian special forces to end the crisis, with 107 foreign hostages and 685 Algerian hostages freed.
However, Minister of Communication Mohamed Said said this would rise when final numbers were issued in the next few hours. "I am afraid unfortunately to say that the death toll will go up," Said was quoted as saying by the official APS news agency.
Details are only slowly emerging on what happened during the siege, which marked a serious escalation of unrest in northwestern Africa, where French forces are battling Islamist militants across the Sahara desert in Mali.
In London, Prime Minister David Cameron said that British citizens were among the dead. "Tragically we now know that three British nationals have been killed and a further three are believed to be dead and also a further British resident is also believed to be dead," Cameron said in a televised statement.
One Briton had already was confirmed killed when the gunmen seized the hostages before dawn on Wednesday at the plant, run by Norway's Statoil along with Britain's BP and Algeria's state oil company.
The Islamists' attack has tested Algeria's relations with the outside world, exposed the vulnerability of multinational oil operations in the Sahara and pushed Islamist radicalism in northern Africa to centre stage.
French Foreign Minister Laurent Fabius defended Algeria's handling of the crisis, after some governments expressed frustration that they had been kept in the dark.
"What everyone needs to know is that these terrorists who attacked this gas plant are killers who pillage, rape, plunder and kill. The situation was unbearable," Fabius said.
"It's easy to say that this or that should have been done. The Algerian authorities took a decision and the toll is very high but I am a bit bothered ... when the impression is given that the Algerians are open to question. They had to deal with terrorists," he told Europe 1 radio in an interview.
The Islamist militants seized the remote compound near the Libyan border, taking a large number of hostages. Said reported that the militants had six different nationalities and the operation to clear the plant of mines laid by the hostage-takers was still under way.
Believed to be among the 32 dead militants was their leader, Abdul Rahman al-Nigeri, a Nigerien close to al Qaeda-linked commander Mokhtar Belmokhtar, presumed mastermind of the raid.
One American has also been confirmed dead. Statoil said five of its workers, all Norwegian nationals, were still missing. Japanese and American workers are also unaccounted for.
Some Western governments expressed frustration at not being informed of the Algerian authorities' plans to storm the complex. Algeria, scarred by a civil war with Islamist insurgents in the 1990s which claimed 200,000 lives, had insisted there would be no negotiation in the face of terrorism.
President Barack Obama said on Saturday the United States was seeking from Algerian authorities a fuller understanding of what took place, but said "the blame for this tragedy rests with the terrorists who carried it out".
British Foreign Secretary William Hague declined to confirm newspaper reports that some hostages had been executed in cold blood. "We don't know that definitively. That sort of thing is quite likely to have happened," he said, noting "the nature of the terrorists involved".
BP's chief executive Bob Dudley said on Saturday four of its 18 workers at the site were missing. The remaining 14 were safe.
PLANNED BEFORE FRENCH LANDED IN MALI
The militant attack was one of the most audacious in recent years and almost certainly planned before French troops launched the operation in Mali this month to stem an advance by Islamist fighters.
Hundreds of hostages escaped on Thursday when the army launched a rescue operation, but many hostages were killed.
Before the Interior Ministry released its provisional death toll, an Algerian security source said eight Algerians and at least seven foreigners were among the victims, including two Japanese, two Britons and a French national.
The U.S. State Department said on Friday one American, Frederick Buttaccio, had died but gave no further details.
Mauritanian news agencies identified the field commander of the group that attacked the plant as Nigeri, a fighter from one of the Arab tribes in Niger who had joined the Algerian Salafist Group for Preaching and Combat (GSPC) in early-2005.
That group eventually joined up with al Qaeda to become Al Qaeda in the Islamic Maghreb (AQIM). It and allied groups are the targets of the French military operation in Mali.
The news agencies described him as "one of the closest people" to Belmokhtar, who fought in Afghanistan and then in Algeria's civil war of the 1990s. Nigeri was known as a man for "difficult missions", having carried out attacks in Mauritania, Mali and Niger.
The apparent ease with which the fighters swooped in from the dunes to take control of an important energy facility, which produces some 10 percent of the natural gas on which Algeria depends for its export income, has raised questions over the country's outwardly tough security measures.
Algerian officials said the attackers may have had inside help from among the hundreds of Algerians employed at the site.
Security in the half-dozen countries around the Sahara desert has long been a preoccupation of the West. Smugglers and militants have earned millions in ransom from kidnappings.
The most powerful Islamist groups operating in the Sahara were severely weakened by Algeria's secularist military in the civil war in the 1990s. But in the past two years the regional wing of al Qaeda gained fighters and arms as a result of the civil war in Libya, when arsenals were looted from Muammar Gaddafi's army.