* Cash ransom or prisoner release possible motives
* Incident highlights Islamic militants in North Africa
WASHINGTON Jan 17 (Reuters) - Algerian militants planned their hostage-taking attack on a remote desert gas plant well before a French military operation against militants in neighboring Mali, European and U.S. national security officials said on Thursday.
Intelligence indicates that the hostage takers, believed to be members of a breakaway faction of al Qaeda's North African affiliate, Al Qaeda in the Islamic Maghreb (AQIM), had planned to capture the hostages and take them to a hideout where it would be harder to mount a rescue attempt, a European security official said.
Representatives of the hostage takers told a news service in nearby Mauritania that the attack was a response to the French military operation to clear out Islamic fighters who have taken control of a large swathe of territory in northern Mali. The French operation began on Friday, Jan. 11.
However, U.S. and Western security sources said they believed it was more likely that the motivation behind the hostage-taking was to obtain a ransom, most likely in cash, but also possibly to seek a release of militant prisoners.
The standoff began when gunmen stormed the gas facility on Wednesday. They said they were holding 41 foreigners and demanded a halt to the French military operation in Mali.
Twenty-five foreign hostages escaped and six were killed on Thursday when Algerian forces launched an operation to free them at a remote desert gas plant, Algerian sources said, as one of the biggest international hostage crises in decades unfolded.
The operation was said to be continuing, however, and there were conflicting and confused reports from the region.
More than five Americans were believed to be among the hostages along with around 10 Britons, as well as citizens of other countries, Western sources said.
Several Western security sources said that although details about the hostage-taking remain murky, available evidence suggests the attack was too sophisticated to have been organized in the wake of the French operation in Mali.
The sources said it was more likely the hostage-takers had seized on news of that operation as a pretext for their attack.
Bruce Riedel, a former CIA analyst who has advised President Barack Obama on counterterrorism policy, said the hostage-taking could fuel criticism that U.S. authorities should have been paying closer attention to the threat posed by Islamic militants in north Africa.
Some of Obama's political opponents for months have been raising similar questions about the U.S. security and intelligence posture in Benghazi, Libya, where militants launched a deadly attack on two U.S. official installations last Sept. 11.
U.S. and European sources said the reported leader of the Algerian militant faction that took workers from BP and Statoil hostage, Mokhtar Belmokhtar, is a former leading member of AQIM. He either split from, or was thrown out of, the al Qaeda affiliate because he was regarded as too difficult to work with and too interested in self-promotion, they said.
A 2008 State Department cable classified "secret" and made public by WikiLeaks said an associate of Belmokhtar may have been involved in handling two Canadian diplomats taken hostage by AQIM in Mali in December of that year. They were later freed.
The cable also alleged that Belmokhtar allegedly once targeted a German diplomat in Mauritania for kidnapping-for-ransom. The cable said that as of 2008, however, Belmokhtar had "specifically ordered his operatives to avoid targeting Am Cits (American Citizens) for fear of retribution from the government."