MANAMA, June 9 (Reuters) - Pirates in the Gulf of Aden are increasingly operating at night and could expand their raids into the Red Sea, the U.S. Navy said on Tuesday

The Bahrain-headquartered Fifth Fleet said it had warned merchant vessels about changes in pirates' tactics following bad weather and the stepped-up presence of international navies.

A navy statement said a confirmed pirate attack took place at the southern end of the Red Sea at the end of May and ships should be cautious when transiting through the area.

Pirates operating off the coast of Somalia have also expanded their forays into an area stretching to the Seychelles Islands to avoid foreign navy ships in the Gulf of Aden, a shipping lane connecting Asia with Europe.

"Pirates have also recently increased their number of attacks during the hours of darkness, highlighting the need for heightened vigilance of merchant mariners during both day and night time transits through the high risk areas," the U.S. Navy said.

Piracy has flourished in recent months off the coast of Somalia, with seaborne gangs seizing several cargo ships for millions of dollars in ransom. (Reporting by Frederik Richter; Editing by Angus MacSwan)

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