At least four people are dead and two are unaccounted for after a helicopter crashed on its way to an oil platform offshore Angola, Chevron confirmed.

A spokesperson for Chevron confirmed that a helicopter with five passengers and a pilot crashed on its way to the Tombua-Landana facility off the coast of Angola. One of the passengers was employed by Chevron's regional subsidiary, while others were serving as contractors for the company.

Chevron said the helicopter did not reach its destination and search and rescue operations were ongoing.

"The team located the remains of four personnel and continues to search for the two colleagues who are unaccounted for," the spokesperson said in response to emailed questions. "Authorities are working through the identification process and will confirm the names and nationalities of those recovered."

Chevron's subsidiary in the region started oil and gas production last year from the Lianzi field in a unified offshore economic zone straddling the borders of Congo and Angola. The project was the first in the region to start operations for Chevron and the first cross-border development in Africa.

A report from the International Crisis Group found oil reserves in and around border regions in Central Africa could rekindle simmering resentments in the post-colonial era. The lack of clearly defined borders, notably in the Great Lakes region of Africa, poses significant risks to regional stability, the ICG found.

Chevron offered no indication as to what caused the crash. Response crews are on scene and the company said the search is underway for the cockpit data recorder and aircraft debris.

It's the second such incident involving helicopter transport to offshore oil facilities. A helicopter used by Norwegian energy company Statoil crashed in April on its way to the Gulfaks B with 13 people on board.

Statoil said the organization of helicopter safety was complicated because of the number of players involved, each of whom have a varying degree of understanding about their role in the work.