In Amenas gas field
UPDATE THURSDAY 17:30 EST:
Between six and 34 foreign hostages were killed on Thursday in a conflict between militants who had seiged a BP oil field and the Algerian
Several Western hostages were killed by an Algerian helicopter strike.
A British security source told CBS News "the Algerians were firing from helicopters at anything that moved."
An employee at the oil field told CBS that four foreign hostages escaped while Reuters reported that 25 foreigners escaped.
As of 15:48 EST, a senior U.S. official tells Martha Raddatz of ABC News that five US hostages are still unaccounted for, and "this is far from over."
The militants want to leave the country with the remaining hostages to use them as a bargaining tool.
At 17:15 EST, Al Arabiya reports that the Algerian army controls part of hostage site, but the gas facility still surrounded.
Algeria reportedly turned down assistance from the UK and the US, starting the operation without informing the Brits.
British Prime Minister Cameron told Algeria's Prime Minister Abdelmalek Sellal he was "extremely concerned" about the "very grave and serious situation." Japan has asked Algeria to immediately stop operations that are "endangering hostages' lives."
A man identifying himself as a Japanese hostage told Al Jazeera that he and a Norwegian hostage had been wounded by Algerian sniper fire. A French national told France24 that the hostages have been forced to wear explosive belts and that the militants are heavily armed.
The U.S. is largely in the dark despite sending a quick response force to the area on Wednesday night. A senior U.S. official told Raddatz around 09:00 EST that they're "trying to get clarity but just don't know anything for sure."
CBS News national security correspondent David Martin reports that a U.S. drone has arrived over the Algerian gas plant, "giving the U.S. its first look at what is happening."
The hostage situation began on Wednesday when at least 20 jihadists raided the In Amenas oil field 60 miles away from the Algeria-Libya border, killing two foreigners and kidnapping between 20 and 41 foreign hostages from countries including the U.S., UK, France, Romania, Ireland, Malaysia, Japan and Norway.
The attack is the first of its kind on an oil infrastructure in Algeria and one of the biggest international hostage crises in decades.
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