BP, the Norwegian company Statoil and the Algerian state oil company Sonatrach, operate the gas field. A Japanese company, JGC Corp, provides services for the facility as well.
British Foreign Secretary William Hague told BBC radio he had dispatched a team to Algeria to help at the British embassy there.
"Excuses being used by terrorists and murderers who are involved — there is no excuse for such behavior, whatever excuse they may claim," he said. "It is absolutely unacceptable, of course. It is, in this case, the cold-blooded murder of people going about their business. So there is no excuse, whether it be connected to Libya, Mali or anywhere else."
U.S. Defense Secretary Leon Panetta had declared Wednesday that the U.S. "will take all necessary and proper steps" to deal with the plant attack, condemning it as "terrorist attack."