Militants with al-Qaida in the Islamic Maghreb last week stormed the Ain Amenas natural gas facility in eastern Algeria. A response by the Algerian military left a number of hostages and AQIM fighters dead.
Statoil Chief Executive Officer Helge Lund said there were "many fatalities." While 12 of the 17 employees there returned home, the situation remains "unresolved" for five Statoil employees at the site.
Statoil said Monday the situation for missing employees hasn't changed. It was flying its flag at half staff for one week in light of the events.
BP started natural gas production at the facility in 2006. The company said production there was targeted at around 870 million cubic feet per day, making it the largest facility of its kind in Algeria.
BP Chief Executive Officer Bob Dudley said 14 of the 18 members of the BP staff in Algeria are safe. He said the severity of the attack is a new experience for BP.
"We have not experienced an attack on any such facilities on this scale before," he said in a statement.
The workforce at Ain Amenas is estimated at around 600 people, said BP. AQIM attacks were said to be a response to a French military intervention in neighboring Mali.
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