Al-Qaida militants stormed the Ain Amenas natural gas facility last week in eastern Algeria. Dozens of people were killed when Algerian military forces retook the facility.
BP said 14 of the 18 employees there are safe and out of the country. BP Chief Executive Officer Bob Dudley said the company was in close contact with the families of the four other employees.
"It is with great sadness that I now have to say that we fear the worst for them all," he said in a statement.
The natural gas project is a joint venture between BP, Norwegian major Statoil and state oil and natural gas company Sonatrach. Statoil had said the situation for five of its 17 employees there "remains unresolved."
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 remains committed to operating in Algeria where it has high quality assets and has been present for over 60 years," the company said in a statement.