Three South African navy personnel have died and a senior officer is in a critical condition after seven members of a submarine crew were swept off the vessel’s deck by large waves as a helicopter attempted a “vertical transfer” of supplies, the Department of Defence said on Thursday.

The accident happened on Wednesday as an Air Force Lynx helicopter was attempting what is known as a “vertrep” – or vertical replenishment – of supplies to the SAS Manthatisi submarine on the ocean surface off the coast of Cape Town, the department said.

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 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.

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.

Cape Town and other areas on South Africa’s south coast have been hit by extremely rough seas since last weekend, caused by a phenomenon known as “spring tide”.

South African armed forces analyst Dean Wingrin told the defenceWeb website that the submarine was moving to the Cape Town waterfront for a three-day navy exhibition starting on Saturday.

The navy has planned the exhibition to mark South Africa’s Heritage Day national holiday on Sunday.

The SAS Manthatisi is one of three German-built Type 209/1400 Heroine-class submarines in the South African navy fleet and had returned to the water earlier this year after undergoing maintenance work.