Three South African navy personnel have died after seven members of a submarine crew were swept off the vessel’s deck by large waves, the country’s department of defence has said.

The South African National Defence Force, which comprises all the armed forces, said a female officer with the rank of lieutenant commander was among the three who died.

A senior officer is in a critical condition after the tragedy which happened as a helicopter attempted a “vertical transfer” of supplies on Wednesday.

The SAS Manthatisi submarine was on the ocean surface off the coast of Cape Town when the accident took place.

The operation was immediately called off and a rescue effort was launched.

All seven submariners were recovered, but three were pronounced dead.

A crew member from the Air Force Lynx helicopter, who was dispatched as a “surface swimmer” to assist in the rescue operation, was also recovered and is in hospital alongside the four surviving submariners.

The National Sea Rescue Institute and other emergency services were called in to help with the rescue, the department of defence said.

There will be an inquiry into the failed operation and the resulting deaths, the department added.