One American and as many as 30 British are among those missing.

(Reuters) – About 60 foreigners were still being held hostage or missing inside a gas plant on Friday after Algerian forces stormed the desert complex to free hundreds of captives taken by Islamist militants, who threatened to attack other energy installations.

The attack, which plunged capitals around the world into crisis mode, is a serious escalation of unrest in northwestern Africa, where French forces have been in Mali since last week fighting an Islamist takeover of Timbuktu and other towns.

“We are still dealing with a fluid and dangerous situation where a part of the terrorist threat has been eliminated in one part of the site, but there still remains a threat in another part,” British Prime Minister David Cameron told his parliament.

A local Algerian source said 60 foreigners were still in the facility and some were being held hostage, but it was unclear how many and how many might be in hiding elsewhere in the sprawling compound. It was also not known whether some might have been killed and the bodies not found.