BP and Norway's Statoil will withdraw staff from two gas plants in Algeria after an attack by militants on one of the sites in the North African country, the companies said on Monday.

Militants attacked the In Salah gas plant, operated with state-owned Sonatrach, with rockets on Friday, causing no casualties or damage. Al Qaeda's North Africa branch claimed responsibility for the attack.

Algeria's energy infrastructure has been heavily protected by the army, especially since a 2013 attack on the In Amenas gas plant, also operated by BP and Statoil, during which 40 oil workers were killed.

"BP has decided to undertake a phased temporary relocation of all its staff from the In Salah Gas and In Amenas JVs in Algeria over the next two weeks. This decision has been taken as a precautionary measure," the British firm said in a statement.

Statoil said it would also withdraw staff from the In Salah and In Amenas plants, together with staff from its operations centre at Hassi Messaoud.

"It will happen over the next few weeks. Those who are on rotation now will not be replaced when they finish their shifts," a Statoil spokesman said, declining to say for security reasons how many employees would be affected.

"It's only been four days since shots were fired at In Salah. The production started again, but in the current situation we believe that this is the right decision to make," the spokesman added.

According to BP's website, In Salah started production in 2004 from the Krechba, Teguentour and Reg fields. In February, it announced the start up of development of the Gour Mahmoud, In Salah, Garet el Befinat and Hassi Moumene fields, to bring output to 9 billion cubic metres a year.

Statoil, BP and Sonatrach were due to restart the third and final processing train at the In Amenas gas plant, damaged during the 2013 attack, later this year.

Statoil repeated on Monday the restart of that train would still happen "in the coming months". – Reuters