Two Americans escaped unharmed Thursday from a hostage standoff with an Al Qaeda-linked group as Algeria's state news agency reports special forces launched a rescue operation to free the remaining foreign hostages. But casualties have been reported and dozens remain unaccounted for.

The Associated Press reports at least six people, and perhaps many more, were killed -- Britons, Filipinos and Algerians. Dozens more remained unaccounted for, including Britons, French, Norwegians, Romanians, Malaysians, Japanese, Algerians, one American and the fighters themselves.

The two Americans who managed to escape the Ain Amenas gas plant are en route to London, sources told Fox News.

A senior U.S. official tells Fox News that at least one unarmed U.S. drone is patrolling the Ain Amenas gas plant to provide intelligence on the situation. At least 20 gunmen attacked and took over the vast complex early Wednesday, reportedly in retaliation for France's military intervention against Al Qaeda-linked rebels in neighboring Mali, though Fox News sources say the attack may have been planned much earlier.

With the hostage drama entering its second day Thursday, Algerian security forces moved in, first with helicopter fire and then special forces, according to diplomats, a website close to the militants, and an Algerian security official. The government said it was forced to intervene because the militants were being stubborn and wanted to flee with the hostages.

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