By Lamine Chikhi
ALGIERS | Fri Jan 18, 2013 12:00pm EST
(Reuters) - More than 20 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.
Thirty hostages, including at least seven Westerners, were killed during Thursday's assault, along with at least 18 of their captors, said an Algerian security source.
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.