DUBAI - Saudi Arabia, the world's top oil exporter, sold $10 billion in a three-part bond issue on Tuesday, taking advantage of a window of opportunity to tap debt markets amid strong investor appetite for Gulf credits.

Pricing on all three tranches of the bond was tightened from initial guidance earlier in the day, as investors piled in with orders. Order books reached over $35 billion ahead of the close.

The kingdom sold $3.25 billion in a five-year tranche at 110 basis points (bps) over U.S. Treasuries (UST), $3.5 billion in 10.5-year bonds at 140 bps over UST, and another $3.25 billion in a 30-year offering at 5.5%, a document from a lead bank showed.

High oil prices helped Saudi Arabia's fiscal balance tilt to its first surplus since 2013 last year, expected to be 2.6% of GDP. A consecutive, albeit narrower, surplus is forecast in 2023, clouded by global economic concerns and an uncertain oil price outlook.

"The timing (of the issue) is a little surprising, given that surplus and indications that the October bonds were pre-financing 2023 maturities," said Justin Alexander, director of Khalij economics and Gulf analyst for GlobalSource Partners.

"But it appears to be opportunistic given a modest dip in yields from the recent peak, which unfortunately was around the time of the October issuance."

The kingdom sold $5 billion in Islamic and conventional bonds in October, which was preceded by a bond buyback as part of its liability management strategy.

BNP Paribas, Citi, Goldman Sachs International, J.P. Morgan, Standard Chartered Bank and Saudi's SNB Capital were mandated on the latest issue.

Several Gulf issuers have taken advantage of improved market sentiment to tap debt markets so far this year, attracting solid order books.

(Additional reporting by Yoruk Bahceli in London and Davide Barbuscia in New York; Editing by Kirsten Donovan and Leslie Adler)