Search and rescue efforts to locate two Chevron contractors who went missing following a natural gas rig fire in Nigeria were called off Thursday.

The contractors were part of a group drilling an exploratory well off the coast of Nigeria. An equipment failure on Monday led to loss of well control and natural gas ignited, engulfing the rig in a fireball that has burned for the past four days, according to a preliminary investigation.

After three days of intensive search and rescue activities for our missing colleagues, I am saddened to report that our efforts have proven unsuccessful and, therefore, we have made the difficult decision to transition to a recovery operation. On behalf of Chevron, we extend our sincere condolences to the families of the missing individuals, said Andrew Fawthrop, managing director, Chevron's Nigeria, Mid-Africa Strategic Business Unit.

It is still uncertain how the fire started. Plans to drill a relief well are ongoing.

Of the 154 people working on the rig and an adjacent barge, all but the two were safely evacuated.