FREETOWN, Nov 27 (Reuters) - Sierra Leone's navy killed three pirates and captured seven others when forces foiled a raid on a South Korean fishing boat off Africa's Atlantic coast, navy and police said on Thursday.

Piracy in African waters is a major concern for shipping firms, several of which have recently diverted vessels away from the Gulf of Aden, a key maritime corridor on the opposite coast to Sierra Leone, after a spate of attacks by armed Somalis.

"I received an SOS call from ... the owner of a trawler at 0400 local time (0400 GMT) on Wednesday saying that his vessel had been attacked," Sierra Leone Navy Captain Danie Mansaray said. After a four-hour gun battle in which the navy killed three pirates and sank their boat, seven raiders were captured and weapons including automatic rifles seized, Mansaray said.

Police said the pirates were in custody and that six of them were from neighbouring Guinea.

Most pirate attacks in African seas take place off the east coast, particularly off Somalia, but shippers are also wary of some parts of West Africa's coast, especially Nigeria.

In the biggest ship hijacking in history, Somali pirates earlier this month captured the Sirius Star, a Saudi supertanker carrying $100 million worth of oil. On Tuesday, a band also from Somalia hijacked a Yemeni ship loaded with steel.

Shipping firms say rerouting cargoes around the tip of South Africa rather than through the Suez Canal will add around 12 days to a standard Gulf-to-Europe voyage and push up freight costs. (Reporting by Christo Johnson; Writing by Daniel Magnowski; Editing by Alistair Thomson)

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