Major oil tankers are avoiding the Suez Canal due to rampant piracy off Somalia.
AP Moller-Maersk of Denmark said on Thursday it is routing some of its 50 oil tankers around the Cape of Good Hope instead.
Intertanko said many other tanker firms were doing the same.
Frontline of Norway, which ferries oil from the Middle East, said it was considering a similar step.
The move follows the capture by Somali pirates of a huge Saudi Arabian supertanker loaded with $US100 million worth of oil on Saturday. It was the biggest ship hijacking in history.
Scores of attacks in Somali waters this year have driven up insurance costs for shipping firms. Diversion of cargo around South Africa risks pushing up prices for manufactured goods and commodities.
Maritime officials say at least three other ships have been hijacked the the seizure of the supertanker Sirius Star. It is now anchored at sea off Gan, 28km east of the port of Haradheere.
Saudi Arabia's foreign minister Prince Saud al-Faisal said on Wednesday the tanker's owners were in negotations over a ransom payment.