Militants aligned with an offshoot of al-Qaida in the Islamic Maghreb stormed the Ain Amenas natural gas facility in eastern Algeria last week, taking a number of Algerians and Westerners hostage. A unilateral move on the installation by the Algerian military left a number of hostages dead.
The facility is operated through a joint venture between BP, state-owned Sonatrach and Statoil.
Helge Lund, chief executive officer for Statoil, said during a town hall meeting in Norway that the "five colleagues are still missing."
The company had 17 employees at the installation at the time of the attack. BP said it had 18 employees on site and that it was "gravely concerned" about four employees who are missing.
"We know there are many fatalities, wounded and affected," Lund said, adding that it was a "difficult and demanding time" for the industry and the international community alike.
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.