Six people were killed when a helicopter chartered by US oil giant Chevron crashed off the coast of Angola, security forces confirmed Tuesday.

The accident happened late Monday near the oil-rich enclave of Cabinda, Captain Armando Nzawisa of the Angolan armed forces, who is heading emergency operations, told AFP.

Four Angolans and two expatriates were killed.

"The helicopter crashed in bad weather en route to a Chevron platform," Nzawisa said.

The bodies of the four Angolans have been recovered but search operations for the two foreigners was still underway, with 19 vessels and three helicopters deployed.

Five of the people killed were Chevron employees. The sixth was the pilot, employed by Angolan company Heli Malongo, whose nationality has not been disclosed.

"We have opened an internal enquiry into the causes of the accident," John Baltz, head of Chevron's Angolan subsidiary, told a press conference.

The helicopter was used as a shuttle between oil platforms and Chevron's onshore facilities in Cabinda, public broadcaster TPA reported.

Cabinda borders the Republic of Congo to the north, and is separated from the rest of Angola to the south by a sliver of territory belonging to the Democratic Republic of Congo.

The province produces 60 percent of Angola's oil, making the country Africa's top oil producer.