The Alexandros T, sank in stormy weather around 285 nautical miles south of the town of Port Alfred.
The vessel was sailing from Brazil to China carrying a load of steel with 33 crew members on board, six of whom have been rescued.
National Sea Rescue Institute spokesman Craig Lambinon told AFP the men are not believed to be in life rafts but are wearing life jackets at the time the ship sank.
"As far as we know only five crew members made it into a life raft. They were rescued together with another crew member floating in the water," Mr Lambinon said.
"The rest are in the water but have their life jackets on and were braving swells last night of between four to five metres," Mr Lambinon said.
The search for other survivors, involving an air force Hercules plane, is underway.
Mr Lambinon said the St Vincent and Grenadine-registered carrier, with a gross tonnage of 91,164, sent out a distress call before it broke up, saying it was taking on water in the heavy swells.
Four of the sailors are Greek, including the captain, his second-in-command and two mechanics, according to a statement issued by the Greek Merchant Marine Ministry in Athens.
The stretch of coastline between Port Elizabeth and the eastern port Durban is notorious for claiming ships, including the carrier Kiperousa which sank there in June last year.
The worst disaster recorded there occurred in 1909 when the SS Waratah went down with all 211 hands on board and in recent years, the cruise liner Oceanos in 1991.