BP and the Norwegian oil company Statoil announced on Monday that they were withdrawing their employees from two of Algeria’s largest natural gas fields after Islamic terrorists staged the second attack in three years on their installations.

In the assault, on Friday, a rocket-propelled grenade attack hit the In Salah natural gas field and processing plant in Krechba, 750 miles south of Algiers. BP, a British company, and Statoil operate the installation jointly with the Algerian national oil company Sonatrach.

The attack on the plant did not result in damage or casualties, according to the companies, and production was interrupted only briefly. The Algerian Army rushed to secure the area, but the episode highlighted the difficulty it has in defending the country’s oil and gas fields, which stretch across a wide swath of the Sahara.

Al Qaeda in the Islamic Maghreb, Al Qaeda’s North Africa affiliate, claimed responsibility soon after the attack on In Salah, Algeria’s third-largest gas field. In an online statement, the group said the assault was part of its “war on the interests of the crusaders.” The group also suggested that it aimed to protect the environment and discourage shale gas exploration.