UNITED NATIONS, Nov 20 (Reuters) - The head of the International Maritime Organization warned on Thursday of "a series of negative repercussions" if ships have to reroute round southern Africa to avoid piracy in the Gulf of Aden.

One of the world's biggest shippers, Denmark's A.P. Moller-Maersk <MAERSKb.CO>, said on Thursday it was sending some tankers around the Cape of Good Hope. A major tanker association, Intertanko, said many others were following suit.

The move follows Saturday's capture by Somali pirates of a Saudi Arabian supertanker loaded with $100 million worth of oil, the biggest ship hijacking in history. There have been dozens of similar previous attacks this year.

Were ships bound from Gulf oil-producing states to Europe to divert round South Africa, "a series of negative repercussions would ensue," IMO Secretary-General Efthimios Mitropoulos told the U.N. Security Council.

A typical voyage from Saudi Arabian oil port Ras Tanura to Gibraltar would almost double in length and take an extra 12 days, delaying replenishment of European and U.S. oil stocks, he said. The current route takes tankers into the Red Sea and through the Suez Canal into the Mediterranean.

Each journey would require an extra 750 metric tons of fuel and emit an extra 2,335 tons of carbon dioxide, Mitropoulos said during a council debate on Somalia.

Freight rates could initially more than double and eventually settle at about 25 to 30 percent higher, he added.

Mitropoulos called on the council to strengthen provisions in resolutions authorizing states to enter Somali waters to combat pirates so that they had "clear rules of engagement", and to urge states that captured pirates to put them on trial.

The Gulf of Aden shipping lane serves more than 12 percent of the total volume of oil transported by sea. (Reporting by Patrick Worsnip; Editing by Peter Cooney)

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