Three South African navy personnel died and a senior officer was in critical condition after seven crew members of a submarine were swept off its deck by big waves as a helicopter was attempting to transfer supplies, the Department of Defence said Thursday.

Wednesday’s accident happened as an Air Force Lynx helicopter was attempting what's known as a vertical replenishment of supplies to the SAS Manthatisi submarine on the ocean surface off the coast of Cape Town, the department said.

Also on Wednesday, four soldiers died and an additional two were critically injured in a separate road crash near the city of Upington. Those deaths happened when two SAMIL 50 army trucks, one towing the other, rolled because of a burst tire. The soldiers were thrown out and one of the trucks landed on them, the department said in a separate announcement.

The two deadly accidents happened approximately three hours apart. It’s rare for there to be multiple deaths in South African armed forces accidents, let alone for two such deadly accidents to happen on the same day.

The submarine operation was immediately called off and a rescue effort was launched after the navy personnel were swept into the sea.

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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 also was recovered and is in the 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 said.

The South African National Defence Force, which comprises all of the armed forces, said a female officer with the rank of lieutenant commander was among those who died. The victims were named after their families were informed.

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.