DUBAI, Oct 9 (Reuters) - National Bank of Abu Dhabi NBAD.AD said on Sunday it had raised US$621 million by placing a 30-year unsecured, zero-coupon, callable Formosa bond issue with institutional investors.

The size was increased from an initial targeted benchmark of $250 million to $300 million after strong investor interest, it said.

Credit Agricole and HSBC were joint book runners with Cathay United Bank acting as co-manager. Citigroup and NBAD acted as structuring agents.

Formosa bonds are sold in Taiwan by foreign issuers and denominated in currencies other than the Taiwan dollar. NBAD said its issue was the largest-ever Formosa bond issue from the United Arab Emirates, and the first 30-year Formosa bond from the Middle East and North Africa.

(Reporting by Tom Arnold; Editing by Andrew Torchia) ((Tom.Arnold@thomsonreuters.com; +97144536265; Reuters Messaging: tom.arnold.thomsonreuters.com@reuters.net))